ELEMENTS  OF  GEOLOGY 


BY 

ELIOT   BLACKWELDER 

ASSOCIATE   PROFESSOR   OF   GEOLOGY,    UNIVERSITY   OF   WISCONSIN 

AND 

HARLAN    H.   BARROWS 

ASSOCIATE   PROFESSOR   OF   GENERAL   GEOLOGY  AND    GEOGRAPHY 
UNIVERSITY   OF   CHICAGO 


NEW   YORK  •:•  CINCINNATI  •:•  CHICAGO 

AMERICAN     BOOK    COMPANY 


COPYRIGHT,  1911,  BY 

ELIOT  BLACKWELDER  AND  HARLAN  H.   BARROWS. 
ENTERED  AT  STATIONERS'  HALL,  I  ONDON. 

B.    A   B.    GEOLOGY. 

w.  i>.    3 


PREFACE 

THIS  is  an  elementary  textbook,  not  a  manual  or  reference 
book.  The  authors  have  sought  to  give  the  student  (1)  an 
understanding  of  the  general  principles  and  processes  of  the 
science,  (2)  a  few  of  its  fundamental  facts,  (3)  an  interest  in 
the  subject,  and  especially  (4)  training  in  clear  thinking.  Many 
of  the  mere  facts  of  the  science  will  be  forgotten  presently  by 
most  students.  The  power  and  habit  of  reasoning  logically 
and  of  thinking  clearly  will  be  of  service  to  the  student  in 
meeting  every  problem  of  later  life.  The  constant  aim  has 
accordingly  been  to  make  the  text  explanatory  rather  than 
merely  descriptive,  and  to  appeal  to  the  judgment  rather 
than  to  the  memory.  The  book  has  been  written  with  the 
belief  that,  while  it  is  the  duty  of  the  teacher  to  develop  in  the 
student  the  power  to  reason,  it  is  the  business  equally  of  the 
text.  This  has  determined  even  the  nature  of  the  questions 
asked  at  the  ends  of  most  of  the  Chapters,  and  elsewhere. 
They  are  in  general  not  questions  the  answers  to  which  may  be 
found  in  the  text,  but  questions  which  the  student  may  reason 
out  for  himself,  provided  the  text  has  been  read  with  under- 
standing. 

The  book  departs  from  current  practice  more  or  less  in  the 
arrangement  of  its  material,  and  particularly  in  the  omission 
of  separate  chapters  on  volcanoes  and  earthquakes.  Though 
very  interesting  and  from  some  standpoints  important,  vol- 
canoes, and  especially  earthquakes,  have  been  minor  factors 
in  the  development  of  the  earth,  so  much  so  as  not  to  merit, 
in  the  opinion  of  the  authors,  the  space  commonly  allotted  to 
them  in  textbooks. 

In  the  historical  chapters  "standard  sections"  have  been 
omitted  because  in  general  they  are  of  relatively  local  appli- 

903869 


6  CONTENTS 

PAOE 

V.  THE   WORK   OF  STREAMS     .        .,       .        .        .        .  125 

THE  PROCESSES  OF  EROSION        .         .  •      .        .        .        .125 

FEATURES  DEVELOPED  BY  RIVER  EROSION        .        .        •  137 

STREAM  DEPOSITS 171 

SUMMARY .188 

REFERENCES 189 

VI.  GLACIERS 191 

CHARACTERISTICS  OF  GLACIKRS 191 

THE  GEOLOGICAL  WORK  OF  GLACIERS       ....  208 

THE  WORK  OF  WATERS  ASSOCIATED  WITH  GLACIERS      .  225 

SUMMARY 230 

QUESTIONS 230 

REFERENCES 231 

VII.  OCEANS   AND   LAKES   , 233 

THE  SHORES  OF  THE  OCEAN 237 

OCEAN  DEPOSITS 255 

LAKES 261 

QUESTIONS 271 

REFERENCES  .        .         .         .         .         .        .         .         .         .  272 

VIII.  THE  GREAT  RELIEF  FEATURES  OF  THE  LAND  274 

MOUNTAINS 274 

PLATEAUS 284 

PLAINS 285 

REFERENCES 287 

PART    IT 
HISTORICAL   GEOLOGY 

IX.  HISTORY   OF   THE    EARTH 289 

GROUPS  OF  ANIMALS  AND  PLANTS      .....  292 

FOSSILS  AND  THEIR  USES 302 

QUESTIONS 307 

REFERENCES 307 

X.  ORIGIN   AND   DEVELOPMENT   OF   THE   EARTH    .  308 

THEORIES  OF  ORIGIN 309 

QUESTIONS 316 

REFERENCES 316 


CONTENTS  7 

PAGE 

XT.   THE   ARCHEOZOIC   ERA       .        .        *        .        ."    .  317 

XII.  THE   PROTEROZOIC   ERA     .       •'.        .  ..   .    .    .        .  322 

PROTEROZOIC  ROCKS  OF  THE  LAKE  SUPERIOR  REGION     .  322 

PROTEROZOIC  ROCKS  IN  OTHER  REGIONS    ....  325 

GENERAL  CHARACTERISTICS  OF  THE  PROTEROZOIC  GROUP  327 

LIFE  IN  THE  PROTEROZOIC  ERA  ......  329 

QUESTIONS 330 

XIII.  THE   CAMBRIAN   PERIOD 331 

XIV.  THE   ORDOVICIAN   PERIOD 339 

XV.  THE   SILURIAN  PERIOD       .        .                 ...  349 

XVI.  THE   DEVONIAN   PERIOD  .        .        .        .         ,         .358 

DEVONIAN  IN  THE  WEST 359 

DEVONIAN  IN  THE  EAST 359 

MIGRATIONS  AND  CHANGES  OF  THE  SKA  LIFE  .         .         .  361 

LIFE  ON  LAND 367 

QUESTIONS      ..........  368 

XVII.  THE  MISSISSIPPIAN    P0RIOD 369 

LIFE  OF  THE  MISSISSIPPIAN  SEA 372 

QUESTIONS 375 

XVIII.  THE  PENNSYLVANIAN   PERIOD  ....  376 

LIFE  OF  THE  COAL  SWAMPS 382 

QUESTIONS      ..........  387 

XIX.  THE   PERMIAN  PERIOD 389 

SUMMARY  OF  THE  PALEOZOIC  ERA      .         .         .         .         .  394 

QUESTIONS .396 

XX.  THE   TRIASSIC   PERIOD 397 

LIFE  OF  THE  TRIASSIC          .......  399 

QUESTIONS 404 

XXI.  THE   JURASSIC   PERIOD 405 

LIKE  OF  THE  JURASSIC          .......  409 

QUESTIONS      ,        ,        ,        «        f        ,        ,        ,        ,        ,  413 


8  CONTENTS 


XXII.  THE   COM  ANCHEAN   PERIOD.        .        .        .        .  414 

XXIII.  THE   CRETACEOUS   PERIOD.        .        .        .   r    .  418 

THE  MESOZOIC  ERA  IN  NORTH  AMERICA  .         ...        .  430 

QUESTIONS 432 

XXIV.  THE   TERTIARY  PERIOD         ...  ,433 

LIFE  OF  THE  TERTIARY  PERIOD 442 

QUESTIONS 446 

XXV.  THE   QUATERNARY  PERIOD 448 

THE  GLACIAL  EPOCH  OUTSIDE  OF  THE  ICE  SHEETS          .  457 

ANIMALS  OF  THE  GLACIAL  EPOCH       .....  460 

THE  RECENT  EPOCH      ........  463 

QUESTIONS 465 

INDEX  467 


ELEMENTS   OF   GEOLOGY 


INTRODUCTION 

The  meaning  and  scope  of  geology.  —  Geology  has  to  do 
with  the  history  of  the  earth  and  of  its  inhabitants.  Its  field 
is  so  broad  that  for  the  sake  of  convenience- and  specialized 
study  it  has  been  divided  into  numerous  branches.  Geology 
is  concerned  with  the  different  members  of  the  solar  system 
and  with  other  heavenly  bodies  in  so  far  as  they  yield  evidence 
as  to  the  origin  of  the  earth,  or  affect  the  activities  now  in 
progress  upon  it.  This  division  of  the  general  subject  is  some- 
times called  Astronomic  Geology,  and  is  related  closely  to  the 
science  of  Astronomy.  The  processes  and  agents  at  work 
changing  the  earth  must  be  studied  carefully  by  the  geologist, 
for  they  are  shaping  the  present  chapter  in  the  history  of 
the  earth,  and  an  understanding  of  them  affords  also  a  key 
by  which  much  of  its  earlier  history,  recorded  in  the  rocks, 
may  be  read.  This  phase  of  the  subject  is  Dynamic  Geology, 
and  it  has  common  ground  with  the  special  science  of  Physi- 
ography or  Physical  Geography,  with  Meteorology,  the  science 
of  the  atmosphere,  and  with  other  sciences.  The  study  of  the 
remains  and  impressions  of  the  plants  and  animals  of  past 
ages  that  are  found  in  the  rocks  is  Paleontology;  it  is  really 
the  historical  side  of  Botany  and  Zoology.  Structural  Geol- 
ogy is  concerned  with  the  arrangement  of  the  materials  of  the 
earth.  That  branch  of  geology  which  deals  with  minerals  is 
Mineralogy,  that  which  studies  rocks  is  Petrology;  both  are 
connected  closely  with  Chemistry.  There  are  still  other 
divisions  of  geology,  but  the  ones  mentioned  are  chief,  and 
enough  have  been  enumerated  to  show  that  geology  is  a  very 

9 


10  ELEMENTS  OF  GEOLOGY 

brqad  sci^hcl£  a&il  that  it  is  related  closely  to  various  sister 
sciences^ »  The  Jimits  of  these  many  branches  are  more  or  less 
4?tif&>&L/  and*  of  necessity  they  overlap.  A  thoroughgoing 
study  of  any  one  of  them  requires  more  or  less  knowledge 
of  some  or  all  of  the  rest.  Geology  is  indeed  one  great  unified 
subject,  and  its  branches  are  really  leading  phases  of  the  sub- 
ject, and  not  distinct  divisions.  Little  or  no  attention  is  paid 
to  them  in  this  introductory  survey  of  the  science.  In  Part 
I  the  materials  of  the  earth  and  their  arrangement,  together 
with  the  processes  and  agents  which  affect  them,  and  the 
changes  which  these  processes  and  agents  are  bringing  about 
upon  and  within  the  earth,  are  discussed.  This  may  be  called 
Physical  Geology.  In  Part  II  the  history  of  the  earth  is  out- 
lined briefly  in  the  light  of  the  principles  developed  in  the 
earlier  chapters,  and  the  progress  of  plant  and  animal  life 
through  past  ages  is  sketched.  This  is  Historical  Geology. 

Geologic  processes  and  agents.  —  Throughout  the  earth  in- 
cessant changes  are  going  on,  often  so  slowly,  however,  that 
centuries  are  required  to  make  their  effects  visible.  Rocks 
are  broken,  or  are  bent  into  folds,  some  of  which  appear  on  the 
surface  as  mountain  ridges.  These  highlands  are  attacked 
in  turn  by  wind,  rain,  ice,  and  other  destructive  agencies; 
their  crumbled  substance  is  carried  off  by  streams,  winds, 
and  glaciers,  only  to  be  deposited  elsewhere.  Much  of  the 
detritus  comes  to  rest  finally  in  the  oceans.  There,  other  pro- 
cesses are  at  work  to  bind  the  loose  grains  into  firm  rocks, 
which  may  later  be  elevated  above  the  sea  and  even  be  folded 
into  more  mountains. 

The  processes  of  change  are  most  conspicuous  where  air, 
water,  and  rocks  are  in  contact  with  one  another.  It  is  at  the 
contact  of  air  and  sea  that  waves  are  made,  and  these  in  turn 
help  to  wear  the  land  and  to  assort  the  sand  and  mud  brought 
down  by  many  streams.  Where  air  and  land  meet,  winds  blow 
dust  from  one  place  to  another,  rains  wash  the  soil,  streams 
wear  their  channels,  and  mountain  crags  are  riven  by  the 
expansion  and  contraction  of  the  rocks  and  by  the  expansion 


INTRODUCTION  11 

of  water  freezing  in  cracks.  Beneath  the  surface,  where 
the  rocks  are  partly  or  wholly  filled  with  water,  changes 
are  taking  place  slowly,  as  in  a  great  chemical  laboratory. 
Some  parts  of  the  rock  are  dissolved  out,  leaving  a  spongy, 
crumbling  mass ;  other  parts  are  cemented  tightly  by  min- 
erals left  in  the  pores  and  cracks  among  the  grains.  Still 
deeper,  where  great  pressure  and  heat  are  ever  present,  the 
rock  is  mashed,  welded,  squeezed  into  sheets,  and  molded 
like  plastic  clay.  When  such  rock  is  resurrected  through 
the  wearing  away  of  the  cover,  it  is  found  so  changed  as  to 
bear  little  resemblance  to  its  original  state. 

The  many  processes  of  change  may  be  grouped  under  four 
general  headings.  They  are  diastrophism,  vulcanism,  metamor- 
phism,  and  gradation.  (1)  Diastrophism  includes  all  move- 
ments of  the  earth's  crust  of  whatever  sort.  Some  are  ex- 
tremely slow  and  continue  for  long  periods,  while  others  are 
rapid  and  of  brief  duration.  Some  affect  vast  areas,  and  others 
are  local.  (2)  Vulcanism  comprises  all  processes  by  which 
lava  and  other  volcanic  products  are  forced  to  the  surface 
from  below,  and  by  which  lava  is  moved  from  lower  to  higher 
levels,  even  though  it  does  not  reach  the  surface.  (3)  The 
processes  by  which  rocks  are  changed,  whether  that  change 
results  in  decay  or  in  consolidation,  are  included  under  meta- 
morphism.  (4)  Gradation  covers  all  processes  which  tend  to 
reduce  the  irregularities  of  the  solid  part  of  the  earth.  An 
uneven  surface  may  be  made  level  by  wearing  down  the  high 
places,  or  by  building  up  the  low  ones,  and  so  gradational  pro- 
cesses are  divided  into  two  classes.  Those  which  seek  to  ac- 
complish their  end  by  leveling  down  the  surface  are  called 
degradational  processes,  in  contrast  to  those  which  tend  to  level 
it  up,  called  aggradational  processes.  Both  phases  of  grada- 
tional work  are  done  by  the  atmosphere,  underground  waters, 
streams,  glaciers,  and  by  the  waves  and  currents  of  the  ocean 
and  of  lakes  and  seas.  These  processes  and  agents  are  dis- 
cussed in  subsequent  Chapters. 


PART  I 

PHYSICAL   GEOLOGY 

CHAPTER  I 
THE   COMPOSITION   OF   THE   EARTH 

THE  GREAT  DIVISIONS  OF  THE  EARTH 

THE  great  divisions  of  the  earth  are  the  atmosphere  or  air, 
the  hydrosphere  or  water  portion,  and  the  lithosphere  or  solid 
part  (Fig.  1). 

The  atmosphere.  —  The  atmosphere  is  a  mixture  of  several 
gases.  While  nitrogen. predominates,  the  three  most  impor- 
tant things  in  the  atmosphere,  geologically,  are  oxygen,  carbon 
dioxide,  and  water  vapor.  They  combine  chemically  with  many 
substances  of  the  lithosphere  to  form  new  compounds,  and 
are  especially  important  in  decomposing  surface  rocks  (p.  103). 
The  condensation  of  the  water  vapor  leads  to  the  precipitation 
of  rain  or  snow,  and  makes  possible  the  work  of  running  water 
and  of  ice.  The  work  of  the  atmosphere  in  conditioning  the 
rainfall  is  perhaps  its  greatest  function,  geologically.  So  far 
as  mere  volume  is  concerned,  however,  these  gases  are  of  minor 
importance.  The  water  vapor,  regarded  frequently,  like  dust, 
as  a  foreign  substance  in  the  air,  rather  than  a  constituent  of 
it,  varies  greatly  in  amount  at  different  times  and  places. 
The  carbon  dioxide  makes  about  .03  per  cent  and  the  oxygen 
about  21  per  cent  of  the  air,  or  approximately  one  fifth  by 
volume.  The  remaining  four  fifths  consists  chiefly  of  nitro- 
gen, an  inactive  gas  chemically,  whose  importance  geologi- 
cally is  confined  largely  to  its  mechanical  effects. 

13 


14 


PHYSICAL  GEOLOGY 


The  air  when  in  motion  performs  mechanical  work  of  great 
importance,  especially  in  dry  regions,  transporting  dust  and 
sand,  often  for  great  distances,  and  wearing  exposed  rock 
surfaces  (pp.  86-91).  Wind-formed  waves  bring  about 
important  changes  along  ocean  coasts  and  lake  shores.  The 


FIG.  1.  —  Diagram  showing  the  general  relations  of  the  lithosphere,  hydro- 
sphere, and  lower  atmosphere*. 

atmosphere  also  acts  as  a  blanket,  protecting  the  rest  of  the 
earth  from  the  fierce  heat  of  the  sun  and  preventing  it  from 
cooling  off  rapidly  by  radiation  of  heat.  Winds  distribute  heat 
and  tend  to  equalize  temperatures. 

Although  the  atmosphere  is  known  to  extend  more  than  one 
hundred  miles  above  'sea  level  and  probably  continues  very 
much  higher,  yet  three  fourths  of  the  air  lies  below  the  tops  of 
the  highest  mountains,  and  its  geological  activity  is  confined 
largely  to  its  bottom  portion,  where  it  is  in  contact  with  the 
land  and  the  water. 


THE  COMPOSITION  OF  THE   EARTH 


15 


The  hydrosphere.  —  The  hydrosphere  includes  all  the 
waters  of  the  earth,  —  the  oceans,  seas,  lakes,  streams,  and  the 
water  underground. 

The  oceans  occupy  nearly  three  fourths  of  the  earth's  sur- 
face, and  contain  water  sufficient  to  cover  the  solid  part  of  the 
earth  nearly  two  miles  deep,  were  the  latter  a  perfect  sphere. 
The  oceans  are  all  connected.  If  the  level  of  the  water  in  one 
is  changed,  all  are  affected.  .Streams  wear  the  rocks  over  and 
against  which  they  flow,  and  move  loose  material  to  lower 
levels,  —  much  of  it  to  the  sea.  Together  with  material 
worn  by  waves  from  the  shore,  or  brought  to  the  sea  by  other 
agents,  the  stream- 
borne  waste  of  the 
land  is  spread  out  on 
the  floor  of  the  ocean 
as  layers  of  sediment. 
The  general  effect  of 
the  work  of  the  hydro- 
sphere is  therefore  to 
wear  down  the  surface 
of  the  land,  and  to 
build  up  the  bottom  of 
the  ocean.  The  work 
of  the  waters  beneath 
the  surface  of  the  land 
is  chiefly  chemical. 
Near  the  surface  the 
general  result  is  to 
bring  about  the  de-  FIG. 
composition  of  the 
rocks ;  at  greater  depths,  the  general  effect  is  to  strengthen 
them  by  depositing  material  in  their  pores  and  cracks. 

In  the  waters  of  the  hydrosphere  the  same  gases  which  make 
the  air  are  dissolved,  together  with  many  solid  substances. 
Common  salt  is  dissolved  in  greatest  abundance  in  the  ocean, 
but  the  lime  carbonate  (p.  22)  and  silica  (p.  19)  in  solution 


2.  —  Rock    containing    several    kinds   of 
fossils.     (Photograph  by  Jessup.) 


16 


PHYSICAL  GEOLOGY 


arc  more  important  from  the  geological  standpoint,  since 
they  are  used  by  various  forms  of  ocean  life  for  the  construc- 
tion of  their  shells.  The  shells  of  marine  organisms  have 
frequently  been  embedded  in  the  sediments  derived  from  the 
land,  and  their  remains  or  the  impressions  they  made  (fossils, 
Fig.  2)  constitute  an  important,  though  imperfect,  record  of 
the  life  which  existed  at  the  time  and  place  the  sediments  were 
accumulated. 

The  lithosphere.  —  The  lithosphere,  as  the  name  implies, 
is  composed  of  rock  so  far  as  known ;  it  is  the  solid  portion  of 
the  earth.  As  the  science  of  geology  deals  very  largely  with 
rocks  in  one  aspect  or  another,  it  is  essential  to  study  them 
and  their  arrangement  in  some  detail. 


THE  MATERIALS  OF  THE  LITHOSPHERE 

The  mantle   rock.  —  Loose,  earthy  material    covers  most 

of  the  land.  When  capa- 
ble of  supporting  plant 
life,  this  is  called  soil. 
The  earthy  matter  of  soil 
is  usually  mixed  with 
partly  decayed  vegetable 
matter,  and  then  is  often 
dark-colored,  even  black. 
Soils  are  generally  com- 
posed of  sandy,  clayey, 
or  limy  particles,  or  of 
combinations  of  these  in 
any  proportion.  In  ex- 
cavations for  cellars,  in 
railroad  cuts,  or  in  other 
exposures,  it  may  often 
be  seen  that  the  soil  gives 
place  below  to  material  which,  though  loose,  is  commonly 
coarser,  more  compact,  and  of  different  color.  This  is  the  sub- 


Fu.  3.  —  Decaying  granite  and  resulting 
rock  waste.  The  granite  is  cut  by  a 
dike  (p.  49).  Southeastern  Wyoming. 


THE  COMPOSITION   OF   THE   EARTH 


17 


soil.  The  soil  and  subsoil  have  been  called  mantle  rock,  since 
they  form  a  covering  or  mantle  for  the  underlying  rock,  which 
is  usually  solid.  Since  the  loose  mantle  rock  is  formed  by  the 
decay  and  breaking  up  of  solid  rock,  it  is  also  called  rock 
waste  (Fig.  3).  Soil  which  remains  above  the  solid  rock  from 
which  it  was  derived  is  residual  soil,  in  contrast  to  transported 
soil,  which  has  been  brought  from  its  place  of  origin  to  its 
present  situation  by  some  of.  the  agents  which  transport 

materials  on  the  sur-      ,_ . . 

face  of  the  earth. 
Such  soils  when  de- 
posited by  rivers  are 
alluvial  soils,  and 
when  accumulated 
by  the  wind,  eolian 
soils.  Much  of  the 
mantle  rock  of  Can- 
ada and  of  the  north- 
ern part  of  the  United 
States  was  brought 
to  its  present  posi- 
tion by  the  continen- 
tal glaciers  which 
once  covered  the  re- 
gion. This  ice-trans- 
ported material  is 
called  drift.  The 
mantle  rock  ranges  in 
thickness  from  inches 
to  scores  and,  in  exceptional  cases,  hundreds  of  feet. 

Classes  of  rocks.  —  Any  considerable  amount  of  mineral 
matter  that  has  been  brought  together  by  natural  means 
constitutes  rock.  A  rock  may  contain  material  of  one  kind,  or 
of  several  kinds,  and  may  be  loose,  like  sand,  or  solid,  like 
granite.  Popularly,  one  does  not  speak  of  sand  or  clay  as 
rock,  but  thinks  only  of  the  solid  rocks  as  such. 

B.   &  B.   GEOL. 2 


FIG.  4,  —  Igneous  rock.    El  Capitan,  Yosemite 
Valley. 


18 


PHYSICAL  GEOLOGY 


Although  solid  rocks  are  exposed  only  occasionally  in  the 
interior  of  the  United  States,  as  in  quarries,  mines,  along  the 


FIG.  5.  —  Horizontal  stratified  rocks  and  bedding  planes. 

courses  of  certain  streams,  and  in  a  few  other  situations,  they 
outcrop  (come  to  the  surface)   over  large  areas  in  eastern 


FIG.  6.  —  Metamorphic  rock.     Contorted  gneiss. 
(Young,  Can.  Geol  Surv.) 


Ontario,  Canada. 


THE   COMPOSITION   OF  THE   EARTH  19 

Canada,  among  the  western  mountains,  and  elsewhere.  They 
are  found  to  differ  among  themselves  in  many  ways.  Their 
particles  are  of  different  kinds,  sizes,  and  shapes;  some  of 
them  are  held  together  weakly,  others  firmly.  Some  rocks 
are  arranged  in  distinct  layers,  while  others  are  not.  Since 
these  and  other  differences  are  largely  the  result  of  the  different 
ways  in  which  the  rocks  were  formed,  they  have  been  classi- 
fied in  the  first  instance  on  the  basis  of  origin.  Rocks  formed 
by  the  solidification  of  lavas  are  Igneous  Rocks  (Fig.  4).  Rocks 
formed  by  the  consolidation  of  sediments  are  Sedimentary 
Rocks.  Because  the  latter  are  usually  arranged  in  layers  or 
strata,  they  are  often  called  Stratified  Rocks  (Fig.  5).  If  the 
character  of  an  igneous  or  a  sedimentary  rock  is  radically 
altered,  it  becomes  a  Metamorphic  Rock  (Fig.  6).  The  more 
common  rocks,  and  the  minerals  of  which  they  are  composed, 
are  discussed  below. 


MINERALS 

The  igneous  rock  shown  in  Figure  7  is  made  up  of  many  angu- 
lar particles  of  several  distinct  kinds,  each  of  which  has  its  own 
constant  characteristics.  These  particles  can  be  separated, 
and,  when  treated  in  the  proper  manner,  may  be  divided 
chemically  into  simpler  things.  Some  of  them,  for  example, 
may  be  divided  into  oxygen  and  silicon.  Although  chemists 
have  been  working  with  oxygen  and  silicon  since  their  dis- 
covery, it  has  been  impossible  to  get  any  still  simpler  things 
from  them.  They  are  accordingly  called  chemical  elements. 
While  some  70  elements  are  found  in  rocks ,  only  8  are  impor- 
tant quantitatively.  These  are,  in  the  order  of  abundance  : 
oxygen  (0),  silicon  (Si),  aluminum  (Al),  iron  (Fe),  calcium 
(Ca),  magnesium  (Mg),  sodium  (Na),  and  potassium  (K). 
The  first  two  make  up  three  fourths  of  the  earth's  crust ;  the 
eight,  98.95  per  cent.  The  oxygen  unites  with  the  other  seven 
elements  to  form  the  following  oxides:  silica  (SiO2),  alumina 
(A12O3),  the  iron  oxides  (FeO,  Fe2O3,  and  Fe3O4),  lime  (CaO), 


20  PHYSICAL  GEOLOGY 

magnesia  (MgO),  soda  (Na2O),  and  potash  (K2O).  The  union 
of  silica  with  the  other  oxides  named,  forms  silicates.  In 
similar  manner,  very  many  other  combinations  of  elements, 
both  simple  and  complex,  occur  in  nature.  The  few  elements 
which  exist  free  as  constituents  of  rocks,  together  with  many 
definite  compounds  of  elements  which  naturally  take  the  solid 
form,  are  minerals.  Ice  is  truly  a  mineral  in  this  sense,  but  is 
not  popularly  so  considered.  Thus  combinations  of  elements 
give  rise  to  minerals,  and  aggregations  of  minerals  form 
rocks. 

When  studied  by  themselves  in  larger  pieces,  minerals  are 
found  to  have  certain  definite  characteristics  in  addition  to 
their  nearly  constant  chemical  composition.  When  formed 
under  favorable  conditions,  most  of  them  assume  geometrical 
forms  that  are  constant  for  each  mineral.  Common  salt, 
for  example,  forms  cubes.  The  form  of  any  given  crystal  is 
determined  by  its  internal  structure,  probably  by  the  ar- 
rangement of  the  particles  (molecules)  of  which  it  is  com- 
posed. Even  though  the  external  form  be  marred  or  de- 
stroyed, this  internal  crystalline  structure  remains  the  same 
for  each  kind  of  mineral,  and  may  be  observed  with  the  aid 
of  a  microscope  in  even  the  most  irregular  pieces  of  the  sub- 
stance. 

Other  convenient  differences  which  may  be  used  to  tell  one 
mineral  from  another  are  found  in  their  color,  manner  of 
breaking,  hardness,  and  luster.  Some  minerals  break  with 
a  shelly  fracture,  like  glass;  others  leave  ragged  surfaces, 
while  many  split  more  or  less  perfectly  along  certain  planes, 
and  thus  leave  shiny,  flat  surfaces.  This  relatively  easy 
splitting  in  certain  planes  is  called  cleavage.  It  is  one  of  the 
most  distinctive  features  of  many  minerals. 

Of  the  more  than  800  varieties  of  minerals  that  have  been 
named  and  classified,  fewer  than  50  are  important  either 
geologically  or  commercially,  while  about  8  make  nearly  all 
of  the  common  rocks,  Most  of  the  metals  are  derived  from 
a  few  more. 


THE  COMPOSITION  OF  THE  EARTH  21 

Quartz  1  (silica,  SiOs),  familiar  as  the  chief  constituent  of  sand,  is 
generally  light-colored  and  glassy  in  appearance.  When  pure  it  is 
transparent,  but  various  impurities  give  it  different  colors  and 
special  names.  Quartz  sometimes  forms  crystals,  usually  six-sided 
prisms  capped  with  pyramids.  In  most  rocks,  it  occurs  as  grains 
without  definite  shape.  It  is  a  very  stable  compound  and  is  the 
hardest  of  the  common  minerals.  Quartz  will  scratch  glass  and 
cannot  be  scratched  with  a  knife.  The  cleavage  of  quartz  is  very 
poor;  indeed,  for  all  practical  purposes,  it  may  be  regarded  as  with- 
out cleavage.  It  has  a  glasslike  fracture,  which  is  often  a  great 
help  in  distinguishing  it  in  igneous  rocks.  Igneous  rocks  decay 
when  exposed  to  the  weather,  and  the  loose  products  make  mantle 
rock.  In  this  process  the  more  complex  minerals  are  broken  up, 
their  elements  entering  into  new  and  simpler  combinations  ;  but 
the  quartz  remains  unaltered.  This  loose  material  may  then  be 
washed  or  blown  away,  the  hard  quartz  particles  becoming  efficient 
agents  in  wearing  the  rock  surfaces  with  which  they  come  in 
contact. 

Feldspars.  —  There  are  several  kinds  of  feldspar,  composed  of 
silica  and  alumina,  together  with  potassium,  calcium,  or  sodium. 
The  most  common  variety  is  orthoclase  (KAlSisOs),  or  potash  feld- 
spar, in  which  potassium  is  the  distinguishing  constituent.  Feld- 
spar is  not  quite  so  hard  as  quartz,  but  too  hard  to  be  scratched 
readily  with  a  knife.  The  color  of  feldspar  is  variable,  pale  yellow, 
pink,  and  especially  white  and  red  varieties  being  common.  The 
general  color  of  many  igneous  rocks  is  determined  by  that  of  their 
feldspars.  The  excellent  cleavage  of  feldspar  leaves  flat,  glisten- 
ing faces,  which  often  afford  the  readiest  means  of  distinguishing 
feldspar  from  quartz  in  rock.  Feldspars  are  important  constituents 
of  most  igneous  rocks.  Certain  clays  result  from  their  rather  ready 
decomposition. 

Augite  is  a  silicate  of  lime,  magnesia,  iron,  and  alumina.  It  is 
dark  green  or  black  in  color,  and  crystallizes  in  oblique  rhombic 
prisms.  Augite  crystals  are  short  and  stubby. 

Hornblende  is  very  similar  to  augite  in  chemical  composition. 
Since  the  two  minerals  are  also  much  alike  in  color  and  hardness, 
they  are  easily  confused,  and  when  they  occur  in  small  grains  are 
often  not  distinguishable.  Hornblende  has  two  perfect  cleavages, 
the  surfaces  meeting  at  angles  of  125°  and  55°,  while  in  augite  the 
cleavage  planes  meet  nearly  at  right  angles.  This  difference  helps 

1  It  is  hardly  necessary  to  say  that  the  study  and  identification  of  actual 
specimens  of  minerals  and  rocks  are  absolutely  essential  to  an  understanding 
of  them. 


22  PHYSICAL  GEOLOGY 

greatly  in  distinguishing  the  minerals  when  they  occur  in  large 
crystals. 

Mica  is  a  complex  silicate,  and  may  be  identified  readily  by  the 
fact  that  it  is  the  only  common  mineral  that  splits  or  cleaves  into 
very  thin  (paperlike),  elastic  leaves.  It  is  soft,  and  ranges  in  color 
from  white  to  green  and  black.  A  light-colored  variety  of  mica, 
called  Muscovite,  a  silicate  of  alumina  and  potash,  is  often  used 
(under  the  name  of  isinglass)  in  stove  doors  and  lanterns.  A  dark- 
colored  variety,  an  iron-magnesia-silica  compound,  is  called  biotite. 
Hornblende,  augite,  and  biotite  are  all  iron-magnesia-silica  com- 
pounds, and  are  known  as  ferromagnesian  minerals. 

Calcite  (calcium  carbonate,  CaCO3)  is  the  principal  constituent 
of  limestone.  It  is  scratched  easily  with  a  knife,  and  effervesces 
when  touched  with  acid  ;  by  these  tests  it  may  be  distinguished 
readily  from  feldspar,  which  it  frequently  resembles  in  general  ap- 
pearance. Calcite  cleaves  readily  along  three  planes,  so  arranged 
as  to  make  a  rhombic  pattern. 

Gypsum  (hydrous  calcium  sulphate,  CaSO4  +  2  H2O)  is  a  white 
mineral,  softer  than  calcite.  It  usually  occurs  in  masses  of  small 
grains  or  fibers  in  which  the  crystal  form  is  not  visible,  and  is  fre- 
quently stained  brown  or  gray  by  the  impurities  it  contains. 

Olivine  (a  silicate  of  magnesia  and  iron)  is  a  hard,  glassy  mineral, 
which  may  often  be  recognized  by  its  grass-green  or  bottle-green 
color.  It  rarely  forms  good  crystals  in  rocks,  but  occurs  as  grains 
and  small  masses  without  definite  shape.  The  fracture  is  uneven. 
Olivine  is  rarely  found  in  the  presence  of  quartz. 

Kaolin  (a  hydrous  silicate  of  alumina)  forms  the  basis  of  clay. 
It  is  very  soft,  and  the  individual  particles  are  not  visible.  Pure 
kaolin  is  white  and  is  known  as  "  porcelain  clay  "  because  of  its 
use  in  the  manufacture  of  chinaware. 

Hematite  (iron  oxide,  Fe2Os)  is  steel-gray  and  hard  when  in  the 
pure  crystalline  form,  but  soft,  red  varieties  are  common,  and  con- 
stitute the  most  important  sources  of  iron  ore  in  the  Lake  Superior 
district,  at  Birmingham,  Alabama,  and  in  many  other  places. 
This  mineral  occurs  in  many  igneous  rocks  and  is  the  red  coloring 
matter  of  many  soils  and  of  bricks.  It  gives  a  red  streak  when 
rubbed  with  a  harder  surface,  a  fact  by  which  it  may  be  distin- 
guished from  other  iron  minerals. 

Magnetite  (iron  oxide,  FesO^  is  black,  and  acts  as  a  magnet.  Its 
crystals  are  often  cubes  or  double  pyramids.  Like  hematite,  it  is 
an  important  ore  of  iron.  It  gives  a  black  streak. 

Limonite  (hydrous  iron  oxide,  2  Fe2O3  +  3  H2O)  is  often 
called  brown  hematite.  It  is  found  frequently  in  marshes, 


THE   COMPOSITION  OF  THE   EARTH 


23 


and  is  then  also  called  bog  iron  ore.     It  gives  a  yellowish  brown 
streak. 

Of  the  minerals  described  above,  quartz,  feldspar,  hornblende, 
augite,  and  mica  make  up"  the  bulk  of  most  igneous  rocks.  Few 
igneous  rocks  have  a  very  large  amount  of  any  other  mineral. 


IGNEOUS    ROCKS 


As  noted  above,  igneous  rocks  are  the  product  of  the  con- 
solidation of  lavas.  The  character  of  an  igneous  rock  is 
determined  by  (1)  the  chemical  composition  of  the  parent 
lava,  and  (2)  the  conditions  under  which  that  lava  solidified. 


FIG.  7.  —  Granite,  about  %  natural  size.  The  light  parts  represent  crystals 
of  two  kinds  of  minerals,  and  the  dark  spots  represent  crystals  of  other 
minerals.  (Photograph  by  Baker.) 

The  kinds  and  proportions  of  chemical  elements  in  a  lava  deter- 
mine the  kinds  of  minerals  and  their  relative  abundance  in  the 
rocks  derived  from  it.  Under  different  conditions  of  solidi- 
fication, lava  of  a  given  chemical  composition  produces  rocks 
of  very  different  appearance.  The  mineral  grains,  for  ex- 


24 


PHYSICAL  GEOLOGY 


ample,  may  be  large  and  easily  distinguished  (Fig.  7),  or 
minute  and  unrecognizable.  They  may  be  of  the  same  size, 
or  of  very  unequal  sizes  (Fig.  8).  These  are  matters  of  tex- 
ture, rock  texture  having  to  do  with  the  size,  shape,  and  ar- 
rangement of  the  particles  of  a  rock. 


FIG.  8.  —  Porphyritic  texture.     About  %  natural  size. 
(Photograph  by  Baker.) 

Chemical  classes  of  igneous  rocks.  —  Those  which  con- 
tain a  large  proportion  of  silica  (more  than  65  per  cent)  are 
called  acidic  rocks,  because  silica  is  an  acid-forming  oxide 
(uniting  with  water  to  form  silicic  acid).  Of  the  other  lead- 
ing oxides  (p.  19)  none  commonly  form  acids.  Similarly, 
those  igneous  rocks  which  contain  much  less  silica  (less  than 
55  per  cent)  and  a  larger  proportion  of  the  bases  (lime,  soda, 
magnesia,  potash,  etc.)  are  called  basic  rocks.  Most  acidic 
rocks  are  light-colored  if  crystalline,  while  basic  rocks  are 
commonly  dark-colored.  An  intermediate  or  neutral  group  is 
sometimes  recognized,  including  rocks  that  contain  55  to  65 
per  cent  of  silica, 


THE   COMPOSITION  OF  THE   EARTH 


25 


Factors  influencing  the  physical  character  of  igneous  rocks. — 
The  circumstances  under  which  lavas  solidify  vary  greatly, 
and  many  factors  influence  the  texture  of  igneous  rocks. 
Among  the  leading  ones  are  (1)  the  rate  at  which  the  lava 
cools,  (2)  the  fluidity  of  the  lava,  and  (3)  the  pressure  under 
which  it  consolidates.  The  texture  is  influenced  also  by  (4) 
the  chemical  composition  of  the  lava. 

(1)  Lava  is  liquid  rock,  a  solution  in  which  certain  minerals 
are  dissolved  in  others.  The  high  temperature  of  the  lava 
appears  to  make  it  possible  for  the  minerals  to  form  a  mutual 
solution.  As  lava 
cools,  the  point  of 
saturation  of  some 
mineral  present  is 
reached,  and  it  be- 
gins to  take  the 
solid  form.  The 
molecules  of  this 
mineral  tend  to 
collect  and  arrange 
themselves  in  reg- 
ular order,  build- 
ing up  crystals 
having  a  definite 
geometrical  form.  As  cooling  proceeds,  the  point  of  satu- 
ration of  other  minerals  is  reached,  and  crystals  of  other 
kinds  begin  to  form.  With  continued  cooling,  the  entire  mass 
may  become  crystalline.  Lava  is  probably  never  so  fluid  as 
water,  and  is  commonly  rather  stiff  (viscous).  It  clearly  re- 
quires some  time  for  molecules  scattered  throughout  such  a 
liquid  to  come  together  and  form  crystals.  Slow,  regular  cool- 
ing therefore  favors  the  development  of  large  crystals. 
The  resulting  coarse-grained  rocks  are  sometimes  called 
granitoids  (granitelike  rocks).  If  cooling  is  rapid,  the  mass 
is  likely  to  become  solid  before  crystals  have  formed,  or  while 
they  are  still  very  minute.  In  the  former  case  the  rocks  may 


FIG.  9.  —  Obsidian,  or  natural  glass.  Shows  the 
glassy  luster  and  fracture.  About  %  natural  size. 
(Photograph  by  Baker.) 


26  PHYSICAL  GEOLOGY 

have  the  structure  and  luster  of  glass,  and  so  are  called  glassy 
rocks  (Fig.  9).  When  composed  of  very  small  crystals,  the 
rock  may  have  a  dense,  stony  appearance,  rather  than  a  glassy 
luster.  If,  after  certain  minerals  have  crystallized  out,  the 
still  liquid  mass  in  which  they  float  be  cooled  suddenly,  a 
rock  may  result  which  is  partly  glassy  or  stony  and  partly 
crystalline  (Fig.  8). 

The  rate  of  cooling,  then,  is  a  chief  factor  in  the  crystalliza- 
tion of  igneous  rocks.  The  rate  is  influenced  by  several  con- 
ditions, of  which  the  following  are  chief,  (a)  Large  bodies 
of  lava  cool  less  rapidly  than  small  ones,  (b)  Masses  deep 
within  the  earth's  crust  cool  more  slowly  than  those  at  or  near 
the  surface,  (c)  The  rate  at  which  a  body  of  lava  cools  is 
affected  also  by  its  shape.  Thus  a  globular  mass  containing 
the  same  quantity  as  a  thin  sheet  would  cool  far  less  rapidly. 
(What  combination  of  these  conditions  would  most  favor  the 
formation  of  coarse-grained  rocks?  Fine-grained?) 

(2)  The  more  fluid  the  lava,  the  easier  and  the  farther  may 
the  molecules  move,  and  the  larger,  other  things  being  equal, 
may  crystals  grow.     The  mobility  of  the  lava  is  determined 
largely  by  its  temperature  and  composition,  but  partly  by 
the  amount  of  water  vapor  it  contains,  and  to  less  extent  by  the 
presence  of  other  volatile  substances,  such  as  carbon  dioxide 
and  fluorine.     In  addition  to  hindering  the  lava  from  becom- 
ing stiffly  viscous,  (a)  the  waters,  etc.,  lower  the  temperature 
at  which  solidification  occurs  and  so  prolong  the  period  of 
crystal  growth,  and  (6)  some  common  minerals  do  not  form 
save  in  their  presence.     These  substances,  especially  water, 
influence  the  process  of  crystallization  so  greatly  that  they  are 
appropriately  called  mineralizers.     That  many  lavas  contain 
large  quantities  of  water  vapor  is  familiarly  illustrated  by  the 
heavy  rains  which  frequently  attend  volcanic  eruptions,  due 
to  the  condensation  of  escaping  steam. 

(3)  The  direct  effect  of  pressure  upon  texture  is  probably 
not   great.     Most   rocks   contract   on   becoming   solid,   and, 
were  other  things  equal,  lava  would  therefore  solidify  more 


THE   COMPOSITION  OF  THE  EARTH  27 

quickly  under  pressure  than  otherwise.  Accordingly,  pressure 
tends  to  oppose  slow  crystallization  and  the  development  of 
coarse  textures. 

Indirectly,  pressure  affects  texture  greatly  through  its  in- 
fluence upon  the  gases  and  vapors  included  in  the  lava.  As 
lava  rises  in  the  pipe  or  chimney  which  leads  downward  from 
the  crater  of  a  volcano,  the  weight  of  the  overlying  column 
of  lava  becomes  less.  This  relief  of  pressure  is  likely  to  per- 
mit the  explosive  expansion  of  the  steam,  by  which  the  lava 
is  sometimes  blown  into  fine  bits  and  hurled  high  into  the  air. 
Very  fine  material  was  thrown  to  an  estimated  elevation  of 
some  17  miles  during  the  great  eruption  of  Krakatoa  (near 
Java)  in  1883.  Lava  blocks  a  number  of  feet  in  diameter  and 
weighing  tons  are  also  sometimes  ejected,  together  with  much 
material  of  intermediate  size.  In  other  cases  the  lava,  possibly 
because  not  so  heavily  charged  with  steam,  flows  quietly  from 
the  volcanic  vent. 

As  the  lava  at  and  near  the  surface  of  a  flow  solidifies,  the 
gases  expand  readily  without  violent  explosion,  and  the  many 
steam  bubbles  frequently  give  the  rock  an  open,  spongy  tex- 
ture (Fig.  11).  Since  it  cooled  promptly,  such  material  is  often 
glassy.  If  the  lava  becomes  solid  under  great  pressure,  as  at 
the  bottom  of  a  thick  flow  or  when  intruded  into  the  rocks 
deep  below  the  surface,  the  included  gases  cannot  expand 
freely,  and  the  resulting  rock  has  a  compact  texture.  Cooling 
very  slowly,  such  rock  is  apt  also  to  be  coarsely  crystalline. 
The  gases  may  be  confined  by  even  a  thin  covering  which  is 
relatively  impervious  to  them,  a  solid  texture  resulting. 

(4)  It  has  already  been  pointed  out  that  the  chemical  com- 
position of  lava  determines  the  kinds  and  proportions  of  min- 
erals in  the  rock  formed  from  it.  It  may  now  be  noted  that 
this  influences  the  texture  of  the  rock,  for  different  minerals 
form  crystals  of  different  shapes,  so  far  as  their  interference 
with  one  another  while  growing  will  permit.  Furthermore, 
lavas  poor  in  silica,  and  particularly  those  rich  in  iron  and 
magnesia,  retain  their  fluidity  to  much  lower  temperatures 


28  PHYSICAL  GEOLOGY 

than  do  those  containing  much  silica.  Hence  the  former 
lavas  may  produce  coarse-grained  rocks  under  conditions 
where  the  latter  would  give  fine-grained  ones. 

We  are  now  prepared  to  describe  a  few  of  the  more  impor- 
tant igneous  rocks.  The  different  kinds  grade  into  each  other 
without  hard  and  fast  lines. 1 

Distinctly  grained  rocks.  —  These  rocks  have  a  solid  tex- 
ture, are  wholly  crystalline,  and  the  grains  can  be  distinguished 
with  the  unaided  eye.  The  grains  may  be  of  uniform  size 
(large,  medium,  or  small),  or  large  crystals  may  be  scattered 
through  a  ground  mass  of  smaller  ones.  In  the  latter  case,  the 
rock  is  called  porphyry,  and  is  said  to  have  a  porphyritic  texture. 
These  terms  are  applied  also  to  rocks  in  which  distinct  crys- 
tals are  scattered  through  a  glassy  or  stony  ground  mass 
(Fig.  8).  One  way  in  which  porphyritic  texture  develops  has 
been  explained  (p.  26).  The  distinctly  grained  rocks, 
whether  porphyritic  or  nonporphyritic,  may  be  further  classi- 
fied on  the  basis  of  the  minerals  they  contain.  While  there  are 
a  great  many  kinds,  only  a  very  few  of  the  more  important 
ones  can  be  described  here.  All  the  different  varieties  shade 
gradually  into  one  another. 

Granite  (Fig.  7)  is  perhaps  the  most  common  of  the  dis- 
tinctly grained  rocks.  It  always  contains  feldspar  (as  the 
predominant  mineral)  and  quartz,  and  frequently  has  subordi- 
nate amounts  of  other  minerals,  especially  mica.  Descriptive 
names  are  often  employed,  which  indicate  the  leading  sec- 
ondary minerals ;  thus  one  may  speak  of  a  mica-hornblende 
granite.  Granites  have  different  colors,  depending  largely  on 
that  of  the  feldspar,  and  on  the  abundance  of  dark  minerals. 
Gray  and  red  varieties  are  especially  common.  Granite  is  an 
acidic  rock.  Large  crystals  of  feldspar  (less  often  quartz) 
may  be  scattered  through  a  granitic  ground  mass  of  smaller 
(but  distinguishable)  grains,  giving  a  granite-porphyry. 

1  For  this  reason  there  is  no  general  agreement  concerning  the  classifica- 
tion of  igneous  rocks.  The  classification  used  here  differs  from  that  employed 
in  many  other  books. 


THE  COMPOSITION   OF  THE   EARTH  29 

Syenite  (B,  Fig.  10)  is  a  rock  composed  chiefly  of  feldspar, 
with  smaller  amounts  of  the  ferromagnesian  minerals,  partic- 
ularly hornblende,  and  little  or  no  quartz.  It  is  usually  gray 
or  reddish,  and  often  closely  resembles  granite  both  in  color 
and  texture.  Syenite  is  a  neutral  rock.  Like  granite,  it 
may  have  a  porphyritic  texture.  Granite  and  syenite  are 
called  fe Idspathic  rocks  (Fig.  10),  because  feldspar  predominates 
in  both.  Syenite  is  a  much  less  common  rock  than  granite. 

Diorite  is  made  up  chiefly  of  hornblende  and  subordinately 
of  feldspar  (C,  Fig.  10) .  Gabbro  consists  mainly  of  augite,  with  a 
subordinate  amount  of  feldspar.  Diorite  and  gabbro  are  gen- 
erally dark-colored.  They  are  sometimes  not  distinguishable, 
because  it  is  not  apparent  to  the  naked  eye  whether  the  dom- 
inant mineral  is  hornblende  or  augite.  The  rock  may  then  be 
called  dolerite  (meaning  deceptive).  Some  diorites  are  neutral 
rocks,  while  others  are  basic.  Gabbro  is  basic.  Diorite  and 
gabbro  are  widely  distributed  and  common  rocks. 

Peridotite  (D,  Fig.  10)  is  a  basic  rock  composed  entirely  of 
the  dark-colored  minerals  —  olivine,  hornblende,  or  augite. 
These  may  occur  alone  or  in  mixtures.  Both  feldspar  and 
quartz  are  absent.  The  rock  is  black  or  dark  green,  and  is 
much  less  common  than  the  preceding  ones. 

Dense  rocks.  —  Most  or  all  of  the  grains  in  the  rocks  of 
this  class  are  too  minute  to  be  distinguished  by  the  naked  eye. 
When  nonporphyritic,  many  of  these  rocks  have  a  rather  uni- 
form, stony  appearance.  Such  a  rock,  when  dark-colored, 
may  be  called  basalt;  when  light-colored,  felsite.  (The  stu- 
dent must  guard  against  confusing  felsite  with  certain  fine- 
grained sandstones.)  Similar  names  are  used  when  the  tex- 
ture is  porphyritic ;  if  the  ground  mass  is  light-colored,  the 
rock  is  felsite-porphyry,  if  dark-colored,  basalt-porphyry.  Fur- 
ther subdivisions  of  the  porphyries  may  be  made  in  terms 
of  the  minerals  which  form  the  visible  crystals.  Thus  there 
is  quartz-felsite-porphyry,  feldspar-basalt-porphyry,  etc.  The 
light  rocks  of  this  class  are  chiefly  feldspathic,  while  the  dark 
ones  are  mainly  ferromagnesian. 


30 


PHYSICAL  GEOLOGY 


A.  Anorthosite,  all  feldspar.  B.  Syenite,  mostly  feldspar. 


C.  Diorite,  some  feldspar.  D.  Peridotite,  no  feldspar. 

FIG.  10.  —  Contrast  of  f eldspathic  and  f erromagnesian  rocks. 
(Pirsson,  Rocks  and  Rock  Minerals.) 


THE   COMPOSITION  OF  THE  EARTH 


31 


Glassy  rocks.  —  This  class  includes  rocks  composed  wholly 
or  in  large  part  of  glass. 

It  has  already  been  seen  that  rock  formed  at  the  surface  of  a 
lava  flow  is  apt  to  be  more  or  less  filled  with  cavities  formed  by 
gas  bubbles.  Such  rocks  are  sometimes  said  to  have  a  vesicu- 
lar texture  (vesicles  =  cavities).  Pumice  (Fig.  11)  is  a  rock  in 


- 
miim 


FIG.  11.  —  Pumice,  about  \  natural  size.     (Photograph  by  Baker.) 

which  such  cavities  take  up  much  of  the  space,  and  are  divided 
by  very  thin  partitions  of  glassy  material.  Bits  of  pumice  are 
found  distributed  widely  over  the  ocean  floor,  for  they  are  often 
floated  long  distances  before  their  small  pores  become  filled 
with  water,  thus  causing  them  to  sink.  As  the  walls  of  the 
cavities  become  thicker  and  the  material  stony,  pumice 
grades  into  scoria  (Fig.  12).  The  cavities  of  scoriaceous  lavas 
are  sometimes  partly  or  wholly  filled  at  a  later  time  by  depo- 
sition of  minerals  from  solution  in  ground  water  (Fig.  13). 
This,  for  example,  is  one  mode  of  occurrence  of  copper  in 
some  of  the  mines  of  northern  Michigan. 

Obsidian  or  volcanic  glass  (Fig.  9)  is  a  solid,  glassy  rock, 


32 


PHYSICAL  GEOLOGY 


generally  black  in  color.     Its  glassy  condition  signifies  rapid 
cooling,  while  its  compact  texture  means  that  gas  bubbles 


FIG.  12.  —  Scoriaceous  texture.    About  %  natural  size. 
(Photograph  by  Baker.) 

were  not  forming  as  it  solidified.     Obsidian  usually  has  a  com- 
position much  like  that  of  granite.     Pitchstone  is  a  glassy  rock 


FIG.  13. — Amygdules  in  lava.  About  natural  size.  The  material  of  the 
amygdules  was  deposited  from  solution  in  ground  water  in  the  cavities  of  a 
scoriaceous  lava.  (Photograph  by  Baker.) 


THE   COMPOSITION  OF  THE   EARTH 


33 


which  has  a  resinous  surface,  and  is  thought  to  resemble  pitch 
in  appearance.  It  is  variable  in  color,  —  red,  brown,  and 
green  varieties  being  common.  Either  obsidian  or  pitchstone 
may  contain  scattered  crystals  which  can  be  recognized,  giv- 
ing rise  to  obsidian-porphyry  and  pitchstone-porphyry : 

Fragmental  volcanic  rocks.  —  Volcanoes  of  the  explosive 
type  throw  out  material  which  falls  in  solid  fragments. 
These  are  classified  on  the  basis  of  size,  shape,  and  texture. 
Volcanic  ash  is  composed  of  very  small,  glassy  fragments.  It 
sometimes  forms  thick  deposits  about  volcanoes.  Still  finer 
material  constitutes  volcanic  dust.  This  is  scattered  widely  by 
the  winds,  some  slight  amount  probably  having  been  carried 
from  certain  volcanoes  to  all 
parts  of  the  world.  Dust  and 
fine  ashes  from  Iceland  vol- 
canoes settled  in  1783  on  cer- 
tain farm  lands  in  northern 
Scotland  in  such  quantity  as 
to  destroy  crops.  Such  ma- 
terial from  Krakatoa  was 
carried  several  times  around 
the  earth  in  1883.  If  the 
material  is  about  the  size  of 
hickory  nuts  or  medium  coarse 
gravel,  it  is  called  lapilli. 
Cinders  are  made  up  of  angu- 
lar pieces  of  open  texture, 
and,  together  with  lapilli  and 
similar  fragments,  form  many  steep-sided  volcanic  cones  (Fig. 
20) .  Masses  of  lava  which  have  become  more  or  less  rounded 
because  of  rapid  rotation  in  the  air  are  bombs  (Fig.  14) .  They 
vary  from  the  size  of  one's  fist  or  less,  to  a  diameter  of  several 
feet.  Volcanic  breccia  is  a  general  term  applied  to  the  beds  of 
coarser  material  (bombs,  lapilli,  coarse  ashes,  etc.),  which  ac- 
cumulate around  the  vent.  The  dust  and  lighter  ashes  settle 
farther  away  to  form  beds  of  tuff. 


FIG.  14.  —  Volcanic  bombs,  Cinder 
Buttes,  Idaho.  (Russell,  U.S. 
GeoL  Surv.) 


B.   &  B.    GEOL.  — 


34 


PHYSICAL  GEOLOGY 


Summary.  —  The  more  important  points  concerning  ig- 
neous rocks  may  be  summarized  as  follows  :  (1)  Igneous  rocks 
are  formed  by  the  solidification  of  lavas.  (2)  Although  they 
contain  many  minerals,  a  few  minerals  make  up  the  great 
mass  of  the  igneous  rocks.  The  most  important  are  (a) 
quartz,  (6)  feldspar,  (c)  the  ferromagnesian  minerals,  and  (d) 
the  iron  oxides.  (3)  Chemically,  igneous  rocks  may  be  divided 
into  three  great  classes  —  acidic,  neutral,  and  basic.  (4)  The 
physical  character  of  igneous  rocks  is  determined  by  (a)  the 
character  of  the  parent  lava,  and  (6)  the  conditions  under 
which  it  solidified.  (5)  Since  both  the  composition  of  lavas 
and  the  circumstances  attending  their  solidification  vary 
widely,  many  kinds  of  igneous  rocks  result.  Of  these  the  few 
that  have  been  mentioned  are  most  important.  They  are 
classified  in  the  accompanying  table. 

CLASSIFICATION   OF   IGNEOUS   ROCKS  1 
A.   GRAINED,  CONSTITUENT  GRAINS  RECOGNIZABLE.     MOSTLY  INTRUSIVE 


a.    Feldspathic    rocks,    usually 
light  in  color 

6.    Ferromagnesian  rocks,  gener- 
ally dark  to  black 

With  quartz 

With  little  or 
no  quartz 

With  subordi- 
nate feldspar 

Without  feld- 
spar 

Nonporphyritic 

GRANITE 

SYENITE 

DIORITE 
GABBRO 
DOLERITE 

PERIDOTITE    . 

Porphyritic 

GRANITE-POR- 
PHYRY 

SYENITE-POR- 
PHYRY 

DlORITE-PoR- 
PHYRY 

B.    DENSE,  CONSTITUENTS   PARTLY  OR  WHOLLY   UNRECOGNIZABLE.     INTRUSIVE  AND 

EXTRUSIVE 


a.    Light-colored,  usually  feld- 
spathic 

6.    Dark-colored  to  black,  usually 
ferromagnesian 

Nonporphyritic 

FELSITE 

BASALT 

Porphyritic 

FELSITE-PORPHYRY 

BASALT-PORPHYRY 

C.     ROCKS    COMPOSED    WHOLLY    OR    IN    PART    OF    GLASS.       EXTRUSIVE 


Nonporphyritic 


OBSIDIAN,  PITCHSTONE,  PUMICE,  ETC. 


Porphyritic 


OBSIDIAN-PORPHYRY,  PITCHSTONE-PORPHYRY 


D.   FRAGMENTAL  IGNEOUS  MATERIAL.     EXTRUSIVE 


TUFF,  VOLCANIC  BRECCIA 


1  After  Pirsson,  with  slight  modification. 


THE  COMPOSITION  OF  THE  EARTH  35 

The  oldest  known  rocks  are  igneous  rocks,  or  metamorphic 
rocks  which  have  been  produced  from  them.  Since  all 
other  rocks  have  been  formed  directly  or  indirectly  from 
igneous  rocks,  the  latter  have  been  called  the  mother  rocks. 
Igneous  rocks,  or  their  altered  products,  are  thought  to  un- 
derlie all  other  kinds  of  rocks,  and  to  make  up  a  very  large 
proportion  of  the  earth's  mass. 

SEDIMENTAR^  ROCKS 

The  formation  of  sediments.  —  It  has  been  seen  (p.  16) 
that  the  greater  part  of  the  land  surface  is  covered  with  loose 
rock  material  formed  from  the  solid  rock.  This  rock  waste 
varies  greatly  in  size,  ranging  from  fine  clay,  through  sand  and 
gravel,  to  large  pieces  of  rock.  It  is  a  matter  of  common  ob- 
servation that  the  finer  material  is  shifted  frequently  from 
place  to  place.  Winds  blow  dust  in  quantity  from  roadways 
and  the  bare  surfaces  of  fields.  Rain  sometimes  washes  large 
amounts  of  earth  down  the  sides  of  freshly  plowed  hills. 
Streams  are  commonly  made  muddy  in  rainy  weather  by  the 
fine  silt  which  they  carry,  and  they  drag  and  roll  coarser  ma- 
terial, such  as  sand  and  gravel,  along  their  channels.  Since 
water  always  flows  down  slope,  the  material  it  carries  is  also 
moving  to  lower  levels.  And  because  all  the  water  which 
does  not  sink  into  the  ground,  evaporate,  or  stop  in  some  lake 
runs  to  the  sea,  it  follows  that  much  of  the  rock  waste  it  moves 
is  carried  into  the  ocean. 

If  water  from  any  stream  is  evaporated,  a  mineral  residue 
remains.  This  means  that  rivers  are  carrying  mineral  matter 
to  the  sea  in  solution  as  well  as  in  solid  pieces.  The  Thames 
River  of  England  carries  over  a  ton  of  dissolved  matter  to  the 
sea  each  minute  on  the  average.  It  has,  indeed,  been  declared 
that  the  one  great  mission  of  running  water  is  to  get  the  land 
into  the  sea.  The  dissolved  material  is  likely  to  remain  in 
solution  in  the  sea  water  for  a  longer  or  shorter  period,  some 
of  it  indefinitely.  Nearly  all  of  the  sediment  which  is  carried 
to  the  sea  in  the  solid  form  soon  settles  to  the  bottom, 


36  PHYSICAL  GEOLOGY 

the  larger  and  heavier  pieces  first,  the  smaller  and   lighter 
later. 

Offshore  waters  are  frequently  agitated  down  to  the  bottom 
by  winds  and  tides,  the  undertow  (a  from-shore  movement  of 
the  water  which  has  come  in  with  the -waves),  and  by  various 
currents.  The  sediment  on  the  bottom  is  rolled  and  dragged 
about  by  these  movements  of  the  water,  and  is  often  shifted 
long  distances  before  reaching  a  final  resting  place.  Normally 
the  bottom  water  moves  most,  close  to  shore  where  it  is  shal- 
low, and  frequently  only  coarse  material,  such  as  gravel,  comes 
to  rest  there,  all  sand  and  mud  being  swept  away.  Farther 
out  the  quieter  bottom  water  is  able  to  move  only  mud  par- 
ticles, and  drops  any  sand  it  may  have  had.  Still  farther  from 
shore  ^he  bottom  waters  become  so  quiet  with  increasing  depth 
that  even  the  finest  mud  comes  to  rest  upon  the  floor.  Thus 


FIG.  15.  —  Diagram  showing  the  relations  to  one  another  and  to  the  land,  of 
beds  of  gravel,  sand,  and  mud. 

the  stream-borne  waste  from  the  land  tends  to  accumulate  in 
belts  of  gravel,  sand,  and  mud,  which  merge  gradually  into  one 
another  (Fig.  15).  Since  the  depth  of  the  water  and  the 
strength  of  the  waves  and  currents  vary  at  points  equally  dis- 
tant from  the  shore,  different  material  is  likely  to  be  accumu- 
lating at  these  different  places;  traced  alongshore,  gravel 
may  give  way  to  sand  and  sand  to  mud.  Furthermore,  the 
agitation  and  depth  of  the  water  vary  from  time  to  time  at  a 
given  place,  because  of  alternating  storms  and  calms,  high  and 
low  tides,  etc.  Hence  the  character  of  sediments  is  subject  to 
changes  vertically  as  well  as  horizontally,  and  alternate  layers 


THE   COMPOSITION  OF  THE   EARTH  37 

of  gravel,  sand,  and  mud  are  formed  in  the  same  place.  This 
division  into  layers,  or  stratification,  as  it  is  called,  is  the  most 
universal  and  important  characteristic  of  water-laid  beds, 
though  not  confined  to  them. 

Countless  numbers  of  minute  organisms  live  in  the  clear  and 
relatively  quiet  waters  beyond  the  reach  of  abundant  land- 
derived  sediment.  Many  of  these  organisms  take  calcium 
carbonate  or  silica  from  solution  in  sea  water,  and  build  it  into 
their  shells  and  other  hard  parts.  When  they  die,  these 
shells,  etc.,  sink,  and  over  millions  of  square  miles  of  the  ocean 
floor  form  a  deposit,  called  ooze  (Fig.  277,  p.  259).  Near  the 
shores,  these  organic  deposits  are  usually  less  important  than 
the  gravels,  sands,  and  muds  brought  down  from  the  land. 

The  consolidation  of  sediments.  —  Material  in  solution  in 
sea  water  is  sometimes  deposited  among  the  particles  of  the 
sediment,  binding  them  together  to  form  firm,  solid  rocks. 
Furthermore,  the  bottom  sediment  is  under  the  weight  of  the 
overlying  material  deposited  later,  and  this  may  become  effec- 
tive in  pressing  the  particles  closer  together,  though  it  prob- 
ably does  not  aid  greatly  in  making  the  mass  coherent. 

Sea-laid  sediments  may  be  exposed  by  an  elevation  of  the 
ocean  bottom,  or  by  a  lowering  of  the  sea  surface,  and  ground 
waters  containing  minerals  in  solution  may  subsequently 
deposit  material  in  their  pores,  further  cementing  the  rocks. 
Rock  cementation  is  often  a  very  slow  process,  and  coastal 
plains  that  have  emerged  from  the  sea  recently  (as  geology 
measures  time)  are  apt  to  be  underlain  by  beds  of  loose 
material  rather  than  of  solid  rock.  This  is  generally  true  in 
the  Atlantic  Coastal  Plain  of  the  United  States  (Figs.  43  and 
44,  pp.  62,  63). 

Certain  sediments  (particularly  lime  carbonate  oozes)  are 
consolidated  not  only  by  cementation,  but  also,  and  in  some 
cases  chiefly,  by  the  formation  of  minute  interlocking  crystals. 

Chief  kinds  of  sedimentary  rocks.  —  Sedimentary  rocks  are 
formed  from  loose  sediments  by  (1)  cementation,  (2)  crystal- 
lization (in  some  cases),  and  (3)  pressure  (to  slight  extent), 


38 


PHYSICAL  GEOLOGY 


as  indicated  above.     Cemented  gravel  is  conglomerate  (Fig. 
16),  while  if  the  pieces  are  angular  instead  of  roundish,  the 


FIG.  16.  —  Conglomerate.    About  %  natural  size.     (Photograph  by  Baker.) 

rock  is  known  as  breccia  (Fig.  17).  Cemented  sand  is  sand- 
stone. The  common  sandstone  cements  are  lime  carbonate, 
the  iron  oxides,  and  silica.  The  nature  of  the  cement  influences 

the  color  and  strength 
of  the  sandstone. 
Quartzite  is  a  dense  and 
very  hard  rock,  pro- 
duced when  the  pores 
of  a  sandstone  are 
completely  filled  with 
quartz.  The  sand 
grains  of  the  sandstone 
are  worn  fragments  of 
quartz  crystals,  and 
the  quartz  molecules 

FIG.  17. -Quartzitic  Breccia.     (Neal.)  deposited   about  them 

arrange  themselves  in  accordance  with  the  internal  structure 
of  quartz  crystals  (p.  21),  and  seek  to  develop  again  the 


THE  COMPOSITION  OF  THE  EARTH  39 

six-sided  prisms.  Cemented  and  compacted  clay  forms  shale. 
Conglomerate,  breccia,  sandstone,  and  shale,  since  they  are 
made  up  of  fragments  of  older  rocks,  are  often  called  fragmen- 
tal  rocks.  The  remains  of  organisms  that  take  calcium  car- 
bonate from  solution  in  the  waters  to  form  their  shells  become 
limestone  when  cemented  or  crystallized.  Some  limestones 
have  been  formed  by  chemical  precipitation  of  calcium  car- 
bonate. Chalk  is  a  very  soft  limestone  of  fine  texture.  Dolo- 
mite (magnesian  limestone)  is  developed  when  some  considerable 
proportion  of  the  calcium  of  a  limestone  is  replaced  by  mag- 
nesium. This  replacement  may  take  place  long  after  the 
formation  of  the  limestone,  or  while  the  material  of  the  lime- 
stone is  accumulating. 

Flint  is  a  very  compact,  dark  gray,  siliceous  rock.  Chert 
is  an  impure  flint,  usually  of  light  color.  These  rocks  do  not 
in  most  cases  form  extended  independent  beds,  but  occur 
chiefly  in  limestones  in  the  form  of  irregular  masses  and  thin 
layers.  Both  contain  fossils  of  the  siliceous  parts  of  various 
sea  animals,  particularly  sponges  and  protozoans  (p.  294). 
The  silica  was  taken  from  sea  water  by  such  animals,  and  at 
their  death  formed  deposits,  often  scattered  through  other 
sediments.  Subsequently,  some  of  it  was  dissolved  by  ground 
water,  and  redeposited  in  certain  places  where  conditions 
favored.  (See  Concretions,  p.  121.)  While  this  seems  quite 
certainly  to  be  the  origin  of  some  flints  and  cherts,  that  of 
others  is  uncertain. 

The  larger  part  of  the  land  surface  is  covered  with  sedimen- 
tary rocks.  Most  of  these  rocks  are  in  layers  and  contain 
marine  fossils.  For  these  and  other  reasons,  it  is  concluded 
that  such  rocks  are  consolidated  sediments  that  were  deposited 
beneath  the  sea  in  the  same  manner  that  offshore  sediments 
are  now  forming.  This  conclusion  carries  with  it  the  inference 
that  at  some  time  in  the  past  the  ocean  waters  have  covered 
large  areas  which  are  now  land.  Since  the  beds  of  sediment 
now  forming  are  nearly  horizontal,  we  conclude  further  that 
all  sedimentary  beds  originally  had  that  position,  and  that 


40  PHYSICAL  GEOLOGY 

great  departure  from  horizontality  indicates  later  disturb- 
ance. 

Nonmarine  fragmental  rocks.  —  While  the  ultimate  goal 
of  running  water  and  of  the  waste  it  carries  is  the  sea,  much 
material  is  deposited  in  lakes,  along  valley  bottoms,  and  in 
other  situations  on  the  land.  These  sediments,  like  marine 
beds,  may  become  firm  rock  by  cementation.  Beds  formed 
in  lakes  that  have  since  been  destroyed  usually  betray  their 
origin  by  their  form  and  attitude,  and  by  the  fossils  which  they 
contain  (p.  267). 

River  deposits  also  have  distinguishing  characteristics, 
some  of  which  are  suggested  by  Figure  185.  Long,  relatively 
narrow  strips  of  coarse  material  indicate  former  positions  of 
the  shifting  stream  channel,  while  the  broader  layers  of  fine 
material  were  spread  upon  the  flood  plain  by  the  quieter 
waters  of  the  overflow.  Cross-bedding  (p.  54)  and  great 
irregularity  of  stratification  are  among  the  most  characteristic 
features  of  stream  deposits.  Occasionally,  they  contain  river 
and  land  shells.  River  deposits  will  be  considered  in  greater 
detail  in  Chapter  V. 

Other  sedimentary  rocks.  —  Certain  special  classes  of  sed- 
imentary rocks,  some  of  them  very  important,  may  best 
receive  attention  in  later  connections.  These  include  gypsum 
and  rock  salt,  precipitated  from  solution  under  special  condi- 
tions (p.  268),  the  iron  ores  (p.  323),  and  a  few  rocks  formed 
by  organisms,  or  themselves  organic,  like  coal  (p.  379),  to- 
gether with  deposits  made  by  winds  (p.  98)  and  by  glaciers 
(pp.  204,  212). 

Summary.  —  The  more  important  points  concerning  the 
origin  of  sedimentary  rocks  are  the  following  :  (1)  Loose  sur- 
face material  is  being  formed  constantly  by  the  decay  and 
breaking  up  of  solid  rock.  (2)  Various  agents  which  transport 
material  on  the  land,  particularly  rivers,  are  shifting  this  rock 
waste  to  new  situations,  especially  to  the  sea.  (3)  In  the  pro- 
cess of  transportation  and  deposition  it  is  more  or  less  per- 
fectly sorted,  and  beds  of  gravel,  sand,  and  mud  result, 


THE  COMPOSITION  OF  THE  EARTH  41 

(4)  These  sediments  are  cemented  to  form  conglomerate, 
sandstone,  and  shale,  the  principal  classes  of  fragmental 
rocks.  (5)  Limestones  are  formed  from  organic  remains, 
and  sometimes  by  precipitation  from  solution.  (6)  These 
sedimentary  rocks  may  themselves  be  exposed  and  may  decay, 
and  the  resulting  waste  may  be  carried  to  the  sea,  or  other 
lodgment  areas,  to  form  new  sediments  and  rocks,  which  may 
in  turn  experience  a  similar  fate.  In  this  manner  many  gen- 
erations of  sedimentary  rocks  have  been  formed,  and  later 
more  or  less  wholly  destroyed.  Since  all  sedimentary  rocks 
are  formed  from  still  older  rocks,  they  are  sometimes  called 
secondary  rocks. 

METAMORPHIC   ROCKS 

Metamorphic  means  changed,  and  metamorphic  rocks  are 
those  which,  originally  igneous  or  sedimentary,  have  been 
altered  in  composition,  or  in  texture,  or  in  both,  since  they 
were  made.  Metamorphism  may  result  in  the  weakening  and 
decay  of  rocks,  or  it  may  strengthen  and  consolidate  them. 

When  rocks  formed  at  or  near  the  surface  are  buried  deeply 
beneath  later  beds,  they  encounter  conditions  very  different 
from  those  under  which  they  were  made.  They  are  under  the 
great  pressure  of  the  rocks  above,  and  may  also  be  subjected  to 
lateral  compression.  They  are  affected  by  higher  temper- 
atures, and  are  acted  upon  by  ground  waters  that  are  made 
powerful  chemically  by  heat  and  pressure.  In  consequence  of 
these  things,  their  composition  may  be  altered,  —  their  minerals 
changing  into  other  minerals  whose  chemical  composition  is 
more  stable  under  the  new  conditions.  They  may  become  more 
thoroughly  consolidated,  harder,  and  more  crystalline.  They 
may  develop  also  a  sheeted  or  banded  structure  (Fig.  18), 
which  is  distinct  from  the  stratification  of  sedimentary  rocks. 
Similarly,  when  igneous  rocks  that  were  formed  by  the  solidi- 
fication of  lavas  at  great  depths  are  exposed  at  the  surface 
through  erosion,  they  find  entirely  new  conditions.  They 
are  subjected  to  the  influences  of  temperature  changes,  of  the 


42  PHYSICAL  GEOLOGY 

gases  of  the  atmosphere,  of  wind  and  water,  of  plants  and 
animals,  and  of  other  agents.  They  commence  at  once  to 
break  up  and  decay,  their  constituents  forming  new  combina- 
tions suited  to  the  new  conditions.  As  in  the  cases  suggested, 
metamorphic  changes  in  general  are  in  the  nature  of  adapta- 
tions to  a  new  environment. 

Although  metamorphism,  strictly  speaking,  includes  all 
changes  in  all  rocks,  and  may  be  destructive  in  its  effects,  as 
well  as  constructive,  yet  in  common  usage  it  implies  radical 
changes  of  the  latter  type,  in  consolidated  rocks.  Such 
changes  take  place  within  the  earth,  especially  at  great 
depths.  The  processes  of  metamorphism  are  treated  in  later 
pages  (78-83). 

In  working  out  its  physical  history,  it  is  frequently  impor- 
tant to  determine  whether  the  metamorphic  rocks  of  a  given 
region  were  derived  from  igneous  or  from  sedimentary 
rocks.  The  answer  is  sometimes  given  by  bodies  of  unaltered 
or  little  changed  rocks  within  the  metamorphic  rocks.  In 
some  cases,  more  or  less  distorted  pebbles  of  various  kinds 
indicate  that  the  parent  rock  was  a  conglomerate  whose  finer 
material  has  been  changed  greatly,  while  the  larger  pebbles 
were  merely  flattened  and  lengthened.  Or  again,  it  may  be 
possible  to  trace  the  gradation  from  the  metamorphic  rocks, 
through  less  and  less  changed  rocks,  into  the  unaltered  rocks 
of  a  neighboring  area.  The  origin  of  many  metamorphic  rocks 
may  be  detected,  too,  by  microscopic  examination  or  by 
chemical  analysis. 

Gneiss  (pronounced  "nice")  is  a  crystalline  rock,  in  many 
cases  containing  the  same  minerals  and  having  the  same  general 
appearance  as  granite,  except  that  it  is  distinctly  banded 
(Fig.  18),  due  to  the  partial  arrangement  of  unlike  minerals 
in  separate  layers.  The  bands  may  be  bent  and  twisted  in  a 
way  that  suggests  intense  crumpling  (Fig.  6).  Granitic 
rocks  usually  become  gneisses  when  metamorphosed.  Gneiss 
may  be  made,  however,  from  various  other  kinds  of  rock. 

Schist  is  in  general  more  closely  and  regularly  banded  than 


THE  COMPOSITION  OP  THE  EARTH 


43 


gneiss,  and  exhibits  a  strong  tendency  to  split  into  uneven 
leaves  or  plates.     These  plates  are  often  spangled  with  glisten- 


FIG.  18.  —  Banded  gneiss.     (Pirsson,  Rocks  and  Rock  Minerals.) 

ing  flakes  of  mica,  or  with  needles  of  hornblende.  Indeed, 
the  splitting  habit  characteristic  of  schists  is  due  largely  to  the 
presence  of  cleavable  minerals  in  parallel  arrangement.  Many 
varieties  of  schist  are  recognized.  Mica  schist  is  most  common, 
and  consists  chiefly  of  quartz  and  mica,  usually  with  a  sub- 
ordinate amount  of  feldspar.  It  is  often  formed  from  slates 
and  feldspathic  sandstones.  In  hornblende  schist,  leaves  of 
imperfect  hornblende  crystals  are  separated  by  other  minerals, 
in  many  cases  by  feldspar,  quartz,  and  mica.  Basic  igneous 
rocks,  when  metamorphosed,  often  become  hornblende  schists. 
The  latter  may  be  formed  also  from  sedimentary  rocks. 

Slate  is  formed  from  shale  by  compression.  It  is  a  hard,  very 
fine-grained  rock,  not  obviously  crystalline,  usually  dark- 
colored,  and  characterized  by  a  remarkable  cleavage,  often 
so  perfect  that  the  rock  is  quarried  extensively  in  parts  of  New 
England  and  in  other  regions  for  roofing  purposes  (Fig.  19). 
The  formation  of  slate  involves  far  less  change  than  the  develop- 
ment of  gneiss  or  schist. 


44 


PHYSICAL  GEOLOGY 


Marble  is  metamorphic  limestone.  It  represents  an  ad- 
vanced stage  in  the  crystallization  of  calcareous  sediments 
(p.  37).  In  some  marbles  the  fine  grains  of  calcite  have  re- 
formed as  interlocking  crys- 
tals larger  than  those  in  most 
granites,  while  in  other  cases 
the  texture  is  so  fine  that  in- 
dividual grains  cannot  be 
distinguished.  Marble  is 
white  if  formed  from  pure 
limestone,  but  because  of  im- 
purities may  be  of  any  color. 
Carbon  and  other  impurities 
often  form  streaks  or  bands  of 
varying  color,  producing  beau- 
tiful and  odd  effects  on  pol- 
ished surfaces.  Marble  is 
much  used  as  an  ornamental 
building  stone.  There  are 
extensive  quarries  at  various 
points  in  the  East,  particu- 
larly in  Vermont.  Unlike  most  metamorphic  rocks,  pure  mar- 
ble is  without  cleavage.  It  may  be  scratched  easily  with  a 
knife,  and  thus  distinguished  readily  from  sandstone  and 
quartzite,  which  it  may  resemble  in  appearance. 


FIG.  19.  —  Fossiliferous  slate  near 
Townsend,  Mont.  (Walcott,  U.S. 
Geol.  Surv.) 


A.    META-SEDIMENTARY    SERIES 


a.    SEDIMENTS 

b.    SEDIMENTARY  ROCKS 

c.  METAMORPHIC  ROCKS 

Gravel 

Conglomerate 

Gneiss,   and  schists  of  vari- 
ous kinds 

Sand 

Sandstone  and  quartzite 

Various  schists  (especially 
quartz  schist)  from  quartz- 
ite. Mica  schist  from  cer- 
tain sandstones 

Clay 

Shale 

Slate,  and  various  schists 
(especially  mica  schist) 

Calcareous  deposits  (shells, 
etc.) 

Limestone 

Marble 

THE   COMPOSITION  OF  THE   EARTH 


45 


B.    META-IGNEOUS   SERIES 


a.    IGNEOUS  ROCKS 

b.    METAMORPHIC  ROCKS 

Granite,  syenite,  and  other  grained  felds- 
pathic  rocks 

Gneiss 

Felsite  and  acidic  tuffs 

Various  schists 

Diorite,  gabbro,   basalt,  and  other  ferro- 
magnesian  rocks 

Various     schists     (especially     hornblende 
schist) 

Only  the  leading  varieties  of  metamorphic  rocks  have  been 
described.  There  are  many  other  kinds  which  cannot  be 
considered  here.  The  general  relations  of  those  discussed  to 
the  rocks  from  which  they  are  commonly  derived  are  shown 
in  the  preceding  table. 

The  relation  of  rocks  to  one  another.  —  At  the  very  outset 
the  student  is  likely  to  encounter  rocks  that  cannot  be  iden- 
tified readily  with  any  of  the  kinds  enumerated  in  the  pre- 
ceding pages.  Thus,  a  rock  may  be  found  which  contains 
both  sand  and  calcite  in  perhaps  nearly  equal  proportions,  and 
which  therefore  combines  the  features  of  a  sandstone  and  a 
limestone.  Varieties  of  rocks  are,  in  fact,  not  definite  species, 
as  are  most  kinds  of  animals  and  plants.  Rather,  they  grade 
into  each  other  by  imperceptible  stages.  By  a  gradual  de- 
crease in  quartz,  granite  verges  toward  syenite.  By  an  in- 
crease in  hornblende  and  a  decrease  in  feldspar,  syenite  passes 
into  diorite.  By  a  decrease  in  the  size  of  its  pebbles,  con- 
glomerate approaches  sandstone.  Similar  transitions  occur 
between  all  related  varieties  of  rocks. 

Furthermore,  rocks  change  after  they  have  been  made, 
and  this  produces  further  gradations  from  one  kind  to  another. 
Thus,  as  noted  above,  granite  may  be  slowly  altered  into 
gneiss,  and  shale  into  slate,  and  slate,  in  turn,  to  schist.  Shale 
and  schist  are  distinct  in  appearance  and  constitution,  yet 
all  possible  gradations  may  be  found  between  them.  It  is 
evident,  then,  that  rock  names  must  be  used  loosely,  and  that 
there  are  few  sharp  dividing  lines  anywhere  in  the  classifica- 
tion, 


46  PHYSICAL  GEOLOGY 

ORIGINAL  STRUCTURES  OF  ROCKS 

By  rock  structure  is  meant  the  mode  of  occurrence  of  rocks, 
the  shapes  of  rock  bodies,  and  the  position  or  attitude  of  those 
bodies.  Thus,  to  say  that  certain  rocks  are  stratified  and 
that  the  beds  are  horizontal,  or  tilted,  or  folded,  is  to  state 
phases  of  their  structure.  The  principal  original  structures 
of  rocks  are  discussed  below,  while  some  structures  developed 
by  the  changes  which  take  place  in  the  outer  part  of  the  earth 
are  described  in  the  next  Chapter. 

SURFACE  STRUCTURES  OF  IGNEOUS  ROCKS 

Volcanic  cones.  —  The  greater  part  of  the  material  extruded 
by  volcanoes  accumulates  near  the  vents,  forming  conical  ele- 
vations. These  cones  vary  in  size  and  shape.  They  range 

in  height  from  a  comparatively 
few  feet  up  to  high  mountains 
like  »Mauna  Loa  in  Hawaii, 
whose  summit  is  some  14,000 
feet  above  the  neighboring  sea 
and  about  30,000  feet  above 
the  sea  floor.  The  slope  of  a 
cone  is  determined  by  its  com- 

V2a°.ie7, 

ders  stand  in  steep  piles  (Figs. 

20  and  21),  while  the  more  liquid  lavas  spread  freely  and  build 
cones  whose  sides  in  exceptional  cases  form  angles  of  only 
two  or  three  degrees  with  a  horizontal  plane  (Fig.  22).  Stiffer 
lavas  form  cones  of  intermediate  steepness.  Lava  cones 
consist  of  many  solidified  streams  of  lava  which  flowed  from 
the  vent  in  different  directions  at  different  times,  producing  a 
sort  of  radial  structure.  Most  cones,  like  Vesuvius,  are  built 
of  both  lava  and  fragmental  material,  and  for  various  reasons 
they  are  often  irregular  in  form  and  structure. 
A  majority  of  the  existing  volcanic  mountains  are  near  the 


THE   COMPOSITION  OF  THE  EARTH 


47 


edges  of  the  continents  and  in  the  sea,  though  some  are  far  in- 
land.    With  some  notable  exceptions,  the  active  and  recently 


FIG.  21.  —  Lava  flow  (in  right  foreground)  and  cinder  cones  near  Flagstaff, 
Ariz.     (R.  T.  Chamberlin.) 

extinct  volcanoes  are  arranged  in  lines  or  belts  where  the  earth's 
crust  has  recently  undergone  severe  movement. 

Lava  plateaus.  —  When  lavas  issue  from  long  cracks  (fis- 
sures) in  the  surface,  or  from  numerous  more  restricted  vents, 
they  may  spread  widely  over  the  surrounding  country  before 
solidifying.  The  distance  to  which  the  lava  flows  depends 
upon  how  fluid  it  is,  upon  the  amount,  and  upon  the  character 

ffavna    Loa 


FIG.  22.  —  Profile  of  the  cone  of  Mauna  Loa,  Hawaii.    Vertical  scale  same  as 
horizontal.     (U.S.  Geol.  Surv.) 

of  the  surface  over  which  it  moves.  (What  combination  of 
these  conditions  would  enable  the  lava  to  flow  farthest  ?)  The 
lava  congeals  first  at  the  top,  forming  a  crust  whose  surface 
is  comparatively  smooth  (Fig.  23),  unless  it  is  repeatedly 
broken  by  the  continued  movement  of  the  still  liquid  mass 
beneath.  In  the  latter  case  the  surface  is  extremely  jagged 


48 


PHYSICAL  GEOLOGY 


and  irregular   (Fig.  24).     Successive  fissure  eruptions  may 
build  up  great  plateaus.     Such  a  lava  plateau,  with  an  area 


FIG.  23.  —  Surface  of  a  comparatively  smooth  lava  flow.     Jordan  Craters, 
Oregon.     (Russell,  U.S.  GeoL  Surv.) 
How  may  the  fissures  be  explained  ? 

of  some  200,000  square  miles,  occurs  in  Washington,  Oregon, 
and  Idaho  (Fig.  25).     The  lava  is  locally  4000  feet  in  thick- 


FIG.  24.  —  Margin  of  a  lava  flow,  Cinder  Buttes,  Idaho.  The  broken  condi- 
tion of  the  lava  is  due  to  movement  after  the  outside  had  hardened.  Note 
the  steep  edge  of  the  flow.  (Russell,  U.S.  GeoL  Surv.) 

ness  along  the  Snake  River,  which  flows  through  the  plateau 
in  a  deep  canon.     The  canon  walls  show  that  the  elevations 


THE  COMPOSITION  OF  THE  EARTH 


49 


buried  by  the  lava  floods  were  in  some  cases  mountains  of 
considerable  elevation.  Lava  flows  built  an  even  larger  pla- 
teau in  the  peninsula  of  India. 


FIG.  25.  —  Lava  flows  of  the  Northwest. 
UNDERGROUND    STRUCTURES   OF   IGNEOUS   ROCKS 

Most  of  the  lava  forced  upwards  from  great  depths  fails 
to  reach  the  surface  and  solidifies  underground.  Igneous 
rocks  formed  from  lavas  deep  below  the  surface  are  called 
plutonic  rocks.  Such  rocks  may 
be  exposed  at  the  surface 
through  the  wearing  away  of 
the  rocks  which  overlay  them. 
Indeed,  much  of  the  igneous 
rock  at  the  surface  is  intrusive 
rock.  Intrusions  of  lava  vary 
in  shape  and  in  their  relations 
to  the  inclosing  rock.  These  differences  have  given  rise  to 
special  names.  Lava  hardens  in  cracks  and  fissures  in  the 
rock  to  form  dikes  (Fig.  26).  Dikes  vary  in  thickness  from 

B.  &  B.  GEOL.  —  4 


FIG.    26.  —  Diagram  of  dikes  and 
sills,     d,  dike  ;  s,  sill. 


50 


PHYSICAL  GEOLOGY 


a  few  inches  to  two  or  three  hundred  feet,  and  in  exceptional 
cases  have  a  length  of  scores  of  miles.  In  many  cases  dike  rock 
is  more  resistant  than  the  adjacent  country  rock,  and  hence 

many  dikes  form 
low,  narrow  ridges 
(Fig.  27).  If  softer 
than  the  rock  it 
penetrates,  the  line 
of  outcrop  of  the 
dike  rock  becomes 
a  depression. 

Lava  intruded  be- 
tween rock  layers  in 
wedge-shaped  sheets 
forms  sills  (Fig.  26). 
Sheets  of  lava  ex- 
truded upon  the 
surface  may  later 
be  covered  by  other 
rocks.  Such  sheets 
then  have  the  po- 
sition of  sills,  though 
of  different  origin. 
In  working  out  the 
geological  history  of  a  region,  it  is  sometimes  important  to 
determine  whether  a  given  lava  sheet  which  lies  between  sed- 
imentary beds  is  intrusive  or  extrusive.  If  the  bottom  of 
the  bed  resting  on  the  lava  sheet  has  been  baked  by  the  hot 
lava,  which  origin  may  be  inferred?  If  the  top  of  the  lava 
sheet  is  glassy  and  has  a  vesicular  texture?  If  tongues  of 
igneous  rock  extend  from  the  lava  sheet  into  the  overlying 
rock?  By  these  and  other  observations,  the  problem  may 
usually  be  solved. 

Sills  merge  into  dome-shaped  intrusions.  If  these  merely 
arched  the  overlying  beds,  they  are  called  laccoliths  (Figs.  28 
and  29).  The  Henry  Mountains  of  Utah  are  notable  ex- 


FIG.  27. —  Porphyry   dike   cutting   tuff.     South- 
western Colorado.     (Howe,  U.S.  Geol.  Surv.) 


THE  COMPOSITION   OF  THE  EARTH 


51 


amples.  If  the  sedimentary  beds  were  broken  and  the  broken 
edges  displaced  (faulted),  the  intrusion  is  a  bysmalith.  The 
Spanish  Peaks  of  southeastern 
Colorado  are  an  illustration. 
Deep-seated  intrusions  of  very 
great  size  (often  many  miles 
across)  are  known  as  batholiths. 
Usually  batholiths  are  of  ir- 
regular form. .  Unlike  laccoliths, 

they  do   not   simply   bulge   the    FIG.    28.  — Diagram  of  a   laccolith 

cover,  but  occupy  vast  spaces       with  associated  dikes  and  sills- 
which  have  been  actually  hollowed   out  of  the  preexisting 
rocks.     Whether  this  was  accomplished  by  melting  and  as- 


FIG.  29.  —  Two  Buttes,  Prowers  County,  Colo.  Sandstone  beds  uplifted  by 
a  laccolithic  intrusion.  The  slopes  have  been  modified  by  erosion.  (Dar- 
ton,  U.S.  Geol.  Surv.) 

similating  the  previous  rocks,  or  otherwise,  is  not  known 
definitely.  Such  intrusions  are  of  rather  common  occurrence 

in  eastern  Canada,  in  asso- 
ciation with  very  ancient 
rocks.  Erosion  has  removed 
their  original  covering,  ex- 
posing the  igneous  cores. 
Granite  batholiths  form  the 

FIG.  30.  —  Diagram  of  a  stock.  •>  f  £  ,-, 

central  cores  ot  many  01  the 

great  mountain  ranges.  Certain  bodies  of  intrusive  rock,  ex- 
posed by  erosion  and  rudely  circular  or  elliptical  in  ground 
plan,  are  called  stocks  (Fig.  30).  They  vary  in  diameter  at  the 


52 


PHYSICAL  GEOLOGY 


FIG.  31. 


Diagram  of   a  young  volcanic 
mountain. 


surface  from  a  few  hundred  yards  to  a  number  of  miles,  and  in 
many  cases  increase  in  size  downwards,  their  sides  cutting  ir- 
regularly across  the  surrounding  rocks.  In  New  England, 

eastern  Canada,  and 
other  regions,  many  gran- 
ite stocks  form  hills  be- 
cause of  the  more  rapid 
erosion  of  the  less-re- 
sistant inclosing  rocks. 
Stocks  differ  from  batho- 
liths  chiefly  in  being  very 
much  smaller,  and  from 
laccoliths  and  bysmaliths 
particularly  in  their  rela- 
tions to  the  surrounding 
rocks.  Igneous  rock  oc- 
curring as  laccoliths,  bys- 
maliths, batholiths,  and 
stocks  is  not  n  beds,  has 
no  cleavage,  and  its  crys- 
tals are  without  system- 
atic arrangement.  Accordingly,  it  is  said  to  have  a  massive 
structure. 

Volcanoes  are,  geologically  speaking,  short  lived.  When 
the  volcanic  forces  die 
away  or  find  relief 
through  other  vents,  no 
further  additions  are 
made  to  the  cone  of  a 
volcano  (Fig.  31),  which 
in  time  is  worn  away 
by  the  agents  of  erosion. 
Long  after  it  has  disap- 


FIG.  32. —  Diagram  showing  a  volcanic 
neck  and  several  mesas  (p.  167)  resulting 
from  the  long  continued  erosion  of  a  vol- 
canic mountain  similar  to  that  shown  in 
Figure  31. 


FIG.  33.  —  Volcanic  neck  near  Adair,  south- 
eastern Colorado.  A  cylindrical  mass  of 
basalt  occupies  the  throat  of  an  extinct 
volcano,  and  is  surrounded  by  an  accumu- 
lation of  talus.  (U.S.  Geol.  Surv.) 


peared,     the     resistant 
rock  formed  by  the  slow  solidification  of  the  lava  which  re- 
mained in  the  tube  leading  down  from  the  crater  may  remain 


THE  COMPOSITION  OF  THE  EARTH 


53 


as  an  abrupt,  steep-sided  hill.  These  elevations,  known  as  vol- 
canic necks  or  plugs  (Figs.  32  and  33),  range  in  diameter  from 
a  few  yards  to  a  mile  or  more.  They  may  be  regarded  as  mon- 
uments, marking  the  sites  of  volcanoes  which  died  ages  ago. 
Volcanic  necks  are  known  at  various  points  in  the  West,  espe- 
cially New  Mexico,  in  Scotland,  and  in  many  other  places.  No 
matter  how  resistant  their  rocks,  volcanic  necks  are  them- 
selves finally  destroyed  as  topographic  features,  leaving  as 
perhaps  the  only  record 
of  the  ancient  volcanoes 
the  igneous  rock  occupy- 
ing the  old  tubes  lead- 
ing to  unknown  depths 
below.  There  are  many 
examples  of  this  stage  in 
the  West. 

Columnar  structure.  — 
The  cracking  of  fine  mud 
as  it  contracts  on  drying 
is  a  familiar  phenomenon. 
In  a  similar  way,  some 
lava  cracks  on  cooling, 
sometimes  forming  regu- 
lar columns  (Fig.  34).  FIG.  34. -The  Devil's  Post  Pile  in  the 
These  are  six-sided  in  Sierra  Nevada  Mountains.  Basalt  which 
t  -,  has  split  into  columns.  (Nat.  Geog. 

many  cases,  and  stand  at       Mag.) 
right  angles  to  the  cool- 
ing surfaces.     In  horizontal  sills  and  lava  flows,  therefore,  the 
columns  are  vertical,  while  in  vertical  dikes  they  are  horizon- 
tal.    They  occur,  among  other  places,  in  the  Palisades  of  the 
Hudson,  and  in  Mount  Holyoke,  Massachusetts.     The  cracks 
which  separate  the  columns  are  joints.     Joints  are  not  pecu- 
liar to  igneous  rocks.     They  affect  rocks  of  all  kinds,  dividing 
them  into  blocks  of  various  sizes  and  shapes. 


PHYSICAL  GEOLOGY 


ORIGINAL  STRUCTURES  OF  SEDIMENTARY  ROCKS 

Stratification.  —  It  has  been  noted  (pp.  36-37)  that  sedi- 
ments are  commonly  arranged  in  distinct  layers,  and  that  this 
stratification  is  the  most  important  structural  feature  of 
sedimentary  rocks.  A  layer  may  be  called  a  bed  or  a  stratum 
(plural  strata).  A  group  of  consecutive  layers  composed  of 
the  same  kind  of  rock  is  often  called  a  formation.  Layers 
are  separated  by  more  or  less  pronounced  division  planes, 
known  as  bedding  planes  (Fig.  5).  An  individual  layer  im- 
plies essentially  uniform  conditions  of  sedimentation.  A 
notable  pause  in  deposition,  a  change  in  the  kind  of  sediment, 
or  a  marked  change  in  the  texture  of  the  material  is  indicated 
by  a  new  layer.  The  longer  conditions  remain  constant, 
therefore,  the  thicker  a  given  layer  becomes.  Thickness  of 
beds  is,  however,  only  a  very  rough  measure  of  time,  for  the 
same  material  gathers  at  unequal  rates  at  different  times  and 
places.  Very  thin  beds,  such  as  those  in  shales,  are  termed 
lamince.  Lamination  is  absent  or  inconspicuous  in  pure 

limestones,   and   usually 
pronounced  in  shales. 

Cross-bedding.  —  If  a 
current  (of  water  or  air) 
moves  material  along  a 
surface  which  terminates 
in  an  abrupt  slope,  most 
of  the  material  will  roll 
down  the  slope  and  come 
to  rest.  Coarse  material 
will  rest  at  a  steep  angle, 
and  fine  material  at  a 
gentler  angle.  If  the 
coarseness  of  the  material 
moved  forward  to  the 

slope   varies   frequently,  numerous   inclined   laminae   will  be 
formed.     If,  in  addition,  the  direction  and  strength   of  the 


FIG.  35.  —  Cross-bedded  sandstone,  canon 
of  Virgin  River,  southern  Utah. 
(Fairbanks.) 


THE  COMPOSITION  OF  THE  EARTH  55 

currents  change  frequently,  the  inclined  laminae  will  slope  in 
different  directions,  and  meet  each  other  at  various  angles. 
This  structure  is  called  cross-bedding  or  oblique  lamination 
(Fig.  35).  It  is  especially  characteristic  of  deposits  made  by 
streams,  and  is  found  in  many  wind  deposits  (Fig.  83,  p.  94). 
It  is  developed  also  off  ocean  and  lake  shores,  where  the  water 
is  shallow  enough  to  be  agitated  frequently  at  the  bottom. 
Conglomerates  and  sandstones  are  cross-bedded  more  often 
than  other  kinds  of  sedimentary  rock  (Why?). 

Ripple  marks.  —  The  rhythmical  movement  of  shallow 
waters  often  develops  on  the  bottom  miniature  ridges,  com- 
monly an  inch  or  two  from  crest  to  crest.  Such  ridges  are 
known  as  ripple  marks  (Fig.  36).  Often  they  may  be  ob- 


FIG.  36.  —  Ripple  marks  upon  a  sandy  beach,  at  low  tide.      (Greger.) 
From  which  direction  did  the  waves  which  formed  the  ripple  marks  come  ? 

served  on  the  sandy  beds  of  clear  and  shallow  streams. 
Here  the  rudely  parallel  ridges  extend  crosswise  of  the  current, 
each  having  a  relatively  long  and  gentle  slope  upstream, 
and  a  shorter  and  steeper  slope  on  the  downstream  side. 
Sand  grains  are  rolled  by  the  current  up  the  gentle  slope  to  the 
crest,  whence  they  fall  down  the  steep  slope  into  the  trough. 
By  a  continuation  of  this  process,  the  ridges  shift  slowly 
in  the  direction  of  the  current.  Ripple  marks  are  produced, 
too,  along  lake  shores  and  seacoasts,  particularly  by  undu- 
latory  movements  of  the  undertow,  out  to  depths  of  twenty 
to  thirty,  or  even  more  feet.  (What  things  determine  how 
far  from  shore  they  may  be  formed  ?)  Ripple  marks  may  be 
preserved  in  the  consolidated  sediments,  and  are  especially 


56 


PHYSICAL  GEOLOGY 


common  in  sandstones.      Ripple  marks  are  also  formed  in 

sand  by  wind  (Fig.  89,  p.  97). 

Sun  cracks. — When  the  water  in  roadside  pools  evaporates, 

the  bottom  mud 
shrinks  and  cracks, 
forming  the  familiar 
mud  cracks  or  sun 
cracks  (Fig.  37).  If 
the  clay  particles 
were  of  uniform  size, 
and  drying  equal 
everywhere,  the 
shrinkage  cracks 
would  probably  be 
arranged  in  regular 
FIG.  37.  — Mud  cracks.  (Fairbanks.)  figures,  after  the  man- 

ner of  certain   cooling   lavas    (p.  53).     As  these   conditions 

seldom  hold,  the  cracking  is  usually  irregular.     Sun   cracks 

may  form  extensively  in  sediments  that  are   exposed   along 

seashores  during  low  tide,  in  dry  interior  basins  on  the  smooth 

mud  floors  of  shallow  and 

temporary  lakes  (play as), 

and  about   lake  borders 

and  along  stream  courses 

when  the  water   is  low. 

If  the  sun-cracked  surface 

is  exposed  for  a  sufficient 

time,it  will  harden  enough 

so  that  the  cracks  will  not 

be    washed    out   readily 

by  the  returning  waters, 

which  may  fill  them  with        FIG.  38.  —  Cast  of  sun   cracks  in   sand- 

other    material    and    so 

preserve  them  permanently  (Fig.  38) .  (In  which  of  the  above 
situations  are  the  chances  for  preservation  best?  Why?) 
Shales  contain  sun  cracks  more  often  than  do  other  rocks. 


THE  COMPOSITION  OF  THE  EARTH  57 

Above  features  aid  in  determining  geology  of  past  times.  — • 
One  may  infer  from  the  composition  and  structure  of  sea-laid 
rocks  the  character  of  the  waters  in  which  they  formed  and 
something  of  the  nature  of  the  lands  which  furnished  the 
sediments.  A  conglomerate  or  sandstone  formation  of  ma- 
rine origin  tells  of  shallow,  rather  rough  waters,  and  of 
relatively  high  lands  whose  vigorous  streams  were  able  to 
carry  coarse  material.  Shallow  water  origin  may  be  indicated 
further  by  frequent  alternation  in  the  degree  of  coarseness, 
by  cross-bedding,  ripple  marks,  or  sun  cracks.  If  the  forma- 
tion contains  fossils,  they  are  likely  to  be  the  remains  of  ani- 
mals which  inhabit  water  of  slight  depth.  No  one  formation 
would  be  apt  to  show  all  these  features,  but  many  formations 
show  several  of  them.  A  sea-laid  shale  formation  implies 
bottom  waters  too  quiet  to  carry  away  mud.  The  presence 
of  ripple  marks  would,  however,  record  some  agitation  of  the 
bottom  water,  while  sun-cracked  shales  must  have  gathered 
close  inshore  where  wave  and  current  action  was  weak  and 
streams  did  not  furnish  coarse  sediment.  If  muds  accumu- 
lated extensively  along  the  ancient  shore,  the  adjacent  land 
must  have  been  so  low  that  its  streams  were  sluggish  and 
therefore  unable  to  carry  coarse  material.  The  fossils  of 
many  limestones  represent  organisms  which  live  only  in  clear, 
quiet  waters.  Such  limestones  may  have  formed  close  to 
shore  if  the  land  was  sufficiently  low,  and  protected  from 
wash  by  vegetation. 

In  a  similar  way  the  composition  and  structure  of  non- 
marine  formations  throw  light  on  the  conditions  which  existed 
when  the  rocks  were  forming. 

The  principles  indicated  here  will  be  applied  frequently  in 
the  historical  chapters,  in  determining  the  geography  of 
North  America  at  the  several  stages  of  its  development. 

QUESTIONS 

1.  Acidic  lavas  are  in  general  stiffer  than  basic  lavas.  Which 
should  you  expect  to  be  the  leading  type  in  (1)  lava  flows,  (2)  sills, 
(3)  laccoliths  ? 


58 


PHYSICAL  GEOLOGY 


2.  Which  of  the  two  dikes  in  Figure  39  is  the  older  ?    Reasons. 

3.  What  is  the  age  of  the  lava  sheet  L  (Fig.  40)  in  comparison 

with  the  age  of  the  sedimentary 
beds  S  and  Si,  (1)  if  the  lava 
sheet  is  intrusive,  (2)  if  it  is  ex- 
trusive ¥ 

4.  What  is  the  relative  age 
of  the  dike  and  the  sedimen- 
tary beds  (S,  Si,  and  S2)  in 
Figure  41  ? 


FIG.  39.  —  Diagram  of  dikes. 


5.   Compare    and    contrast 


the  texture  of  the  rock  in  a  thin  and  a  very  thick  dike  ;  at  the  sur- 
face and  in  the  central  portion  of  a  massive  lava  flow. 


FIG.  40.  —  Diagram  of  lava  FIG.    41.  —  Diagram    of    dike 

sheet  between  sedimentary  and   associated  sedimentary 

beds.  beds. 

6.  Did  the  lava  of  the  bombs  shown  in  Figure  14  solidify  before 
or  after  striking  the  ground  ?    Reasons. 

7.  Might  the  fact    that 
a    given    lava  plateau  had 
been    built    up    by   several 
distinct  flows  be  told  by  the 
texture  of  the  rock  ?     If  so, 
how? 

8.  How  do  igneous  rocks 
come  to  be  at  the  surface? 

9.  Coarse-grained   gran- 
ites,   schists,    and    gneisses 
outcrop  at  many  points  in 
the    uplands    of    southern 
New  England.    Where  were 
these    rocks    formed    with 
reference    to    the    surface? 
What   inference,    therefore, 
may  be  made  concerning  the 
amount  of  erosion  which  has 
occurred  in  the  region  ? 

JO.    (1)  What  is  the  rela- 


1 


4         5MILE5 


FIG.  42.  —  Generalized  map  of  small  area 
southeast  of  Port  Orford,  Ore.  Short 
lines  represent  igneous  rocks ;  horizontal 
lines  sedimentary  rocks,  with  some  meta- 
morphics. 


THE  COMPOSITION  OF  THE  EARTH  59 

tive  age  of  the  igneous  and  sedimentary  rock  (Fig.  42)  (a)  if  the  former 
is  extrusive,  (6)  intrusive  ?  (2)  What  hypotheses  may  be  advanced 
to  account  for  the  isolated  areas  of  sedimentary  rock  within  the 
igneous  rock  area  ?  How  could  these  theories  be  tested  in  the  field  ? 
(3)  How  could  one  determine  in  the  field  whether  the  igneous 
rock  is  intrusive  or  extrusive  ? 

REFERENCES 
MINERALS  AND  ROCKS 

DANA  :   Minerals  and  How  to  Study   Them.     2d  ed.     (New  York, 

1902.) 

GEIKIE,  J.  :  Structural  and  Field  Geology.     (New  York,  1905.) 
KEMP  :  A  Handbook  of  Rocks.     2d  ed.     (New  York,  1900.) 
PIRSSON  :  Rocks  and  Rock  Minerals.     (New  York,  1908.) 

VOLCANOES,  ETC. 

BONNE Y  :   Volcanoes;  Their  Structure  and  Significance.     (New  York, 

1899.) 
DILLER  :   Mi.  Shasta,  a  Typical   Volcano,  in  Physiography  of    the 

United  States,  pp.  237-268.     (New  York,  1895.) 

—  The  Geology  and  Petrography  of   Crater  Lake  National  Park; 
Prof.  Paper  No.  3,  U.S.  Geol.  Surv. 

DUTTON  :    Hawaiian    Volcanoes,   in    4th    Ann.    Rept.,    U.S.    Geol. 

Surv.,  pp.  81-219. 
-  Mount  Taylor  and  the  Zuni  Plateau,  in  6th  Ann.  Rept.,  U.S. 

Geol.  Surv.,  pp.  105-198. 
GILBERT  :   Geology  of  the  Henry  Mountains,  pp.  18-60 ;  U.S.   Geog. 

and  Geol.  Surv.,  Rocky  Mt.  Region.     (Washington,  1877.) 
HEILPRIN  :    Mont   Pelee   and   the    Tragedy   of   Martinique.     (Phila- 
delphia, 1903.) 
HILL  :    Report  on  the   Volcanic  Disturbance  in  the   West  Indies,  in 

Nat.  Geog.  Mag.,  Vol.  XIII,  pp.  223-267. 
HOVEY  :    The  Eruption  of  La  Soufricre,  St.  Vincent,  in  May,  1902, 

in  Nat.  Geog.  Mag.,  Vol.  XIII,  pp.  444-459. 
JAGGAR  :    The   Eruption   of  Mount    Vesuvius,    April   7-8,    1906,   in 

Nat.  Geog.  Mag.,  Vol.  XVII,  pp.  318-324. 
JUDD  :    Volcanoes.     (New  York,  1893.) 
RUSSELL  :    Volcanoes  of  North  America.     (New  York,  1897.) 

—  The  Recent   Volcanic  Eruptions  in  the   West   Indies,   in  Nat. 
Geog.  Mag.,  Vol.  XIII,  pp.  267-285. 

—  Volcanic   Eruptions   on   Martinique  and  St.  'Vincent,   in  Nat. 
Geog.  Mag.,  Vol.  XIII,  pp.  415-436. 


60  PHYSICAL  GEOLOGY 

SHALER  :   Aspects  of  the  Earth,  pp.  46-97.     (New  York,  1889.) 
The  topics  of  this  and  later  chapters  are   discussed  at  greater 

length  in  larger  textbooks  and  manuals.      Among   the  best  of  these 

are : 

CHAMBERLIN  AND  SALISBURY  :  Geology.     3  vols.     (New  York,  1904, 

1906.) 
—  College  Geology.     (New  York,  1909.) 

DANA  :    Manual  of  Geology.     4th  ed.     (New  York,  1895.) 

GEIKIE  :    Textbook  of  Geology.     4th  ed.     (London,  1903.) 


CHAPTER  II 
PHYSICAL    CHANGES    OF   THE    OUTER   SHELL 

The  earth's  crust.  —  In  studying  the  solid  part  of  the  earth, 
we  are  necessarily  limited  to  a  thin  shell  near  the  surface. 
In  the  deepest  mines  and  canons  we  may  go  down  to  a  depth 
of  a  little  more  than  a  mile.  By  the  slow  denudation  of  the 
uplifted  lands,  rocks  which  were  once  buried  to  a  depth  of 
several  miles  may  be  uncovered  at  the  surface.  This  outer 
shell,  which  alone  is  open  to  investigation,  is  the  subject  of 
the  present  Chapter.  It  has  often  been  called  the  "  crust  of 
the  earth,"  in  allusion  to  an  older  theory  that  the  interior  is 
so  hot  as  to  be  liquid,  but  is  covered  by  a  thin,  solid  crust. 
Although  this  theory  has  been  largely  abandoned,  the 
term  is  convenient,  and  we  shall  use  it  to  mean  simply 
the  outer  part  of  the  earth,  which  is  partially  open  to  obser- 
vation. 

Surface  features  of  the  crust.  —  The  outside  of  the  earth 
has  an  irregular  surface.  A  glance  at  a  model  of  the  globe 
shows  that  there  are  several  broad,  smooth  tracts,  which  are 
sunk  on  an  average  of  about  two  and  one  half  miles  below  the 
surface  of  the  sea.  These  are  the  great  ocean  basins.  Be- 
tween them  large  plateaus  stand  out  in  relief  (Fig.  43).  The 
so-called  continents  are  merely  the  portions  of  these  plateaus 
that  are  now  out  of  water,  and  hence  are  land.  The  great 
surface  features  of  the  earth  are,  then,  the  oceanic  depressions 
and  the  continental  plateaus. 

Upon  examining  these  major  features  in  more  detail,  we 
find  that  the  surface  of  the  land  is  notably  rougher  than  that 
of  the  sea  bottoms.  On  the  former  we  see  mountains,  ridges, 
and  minor  plateaus,  with  their  complementary  valleys,  basins, 

61 


62  PHYSICAL  GEOLOGY 

and  lowlands  (Figs.  43  and  44).  Some  of  the  basins  contain 
lakes  or  seas,  while  others  do  not.  In  the  oceans  likewise 
there  are  irregularities,  such  as  projecting  islands,  and  the 
hollows  known  as  "  deeps  " ;  but,  on  the  whole,  the  ocean 


FIG.  43.  —  Photograph  of  a  relief  model  of  North  America. 

floors  are  far  less  rugged  than  the  lands.  This  is  due  partly 
to  the  fact  that  streams,  glaciers,  and  other  agencies  which 
roughen  the  land  surface  do  not  operate  in  the  oceans,  and 
partly  to  the  fact  that  the  deposition  of  sediment  upon  the 
sea  bottom  tends  constantly  to  smooth  out  such  irregulari- 
ties as  may  exist. 

Movements   within  the  crust.  —  As   a   matter  of   human 
experience  the  earth  seems  to  be  firm  and  stable  to  the  last 


PHYSICAL  CHANGES  OF  THE  OUTER  SHELL 


63 


degree,  save  for  such  exceptional  and  sudden  disturbances  as 
landslides  and  earthquakes.  But  an  examination  of  the 
rocks  of  almost  any  region  discloses  evidence  of  movements 
which  have  taken  place,  and  there  is  even  proof  that  such 


FIG.  44.  —  Chief  topographic  divisions  of  North  America.    Compare  this 
with  the  photograph  of  the  model  on  the  opposite  page. 

movements  are  continually  recurring.  Indeed,  the  study  of 
geology  can  hardly  fail  to  emphasize  the  fact  that  the  earth  is 
forever  undergoing  changes  of  many  kinds,  which  after  long 
lapses  of  time  produce  great  results.  These  changes  include 
crustal  movements  of  one  kind  or  another.  Some  of  the 


64  PHYSICAL  GEOLOGY 

movements  are  sudden,  like  those  which  produce  earthquakes, 
while  others  are  very  slow. 

The  most  effective  movements  are  the  slow  ones,  —  so 
slow  that  in  comparison  the  hour  hand  of  a  watch  is  revolving 
rapidly.  We  cannot  readily  detect  such  changes  while  they 
are  in  progress,  but  their  results,  after  long  periods  of  time, 
are  obvious.  Slow  movements  of  this  sort  affect  every- 
thing from  whole  continents  to  the  smallest  invisible  particles 
of  rock.  Some  of  them  may  now  be  considered  in  more 

detail. 

Warping  of  the  surface.  —  On  the  slopes  of  Mt.  St.  Elias, 
in  Alaska,  modern  sea  shells  have  been  found  attached  to  the 


FIG.  45.  —  Folded  beds  of  limestone  on  the  south  coast  of  Alaska.     (Stan ton 
and  Martin,  U.S.  Geol.  Surv.) 

rocks  just  as  they  once  grew,  but  several  thousand  feet  above 
the  sea  level.  It  appears  that  the  coast  has  been  slowly 
raised  above  the  sea  to  that  extent.  Conversely,  on  the 
shores  of  North  Carolina,  stumps  of  trees  are  found  stand- 
ing out  in  salt  water,  where  they  did  not  grow.  From  this 
it  becomes  evident  that  either  the  land  has  gradually  sunk 
beneath  the  sea,  or  the  sea  has  risen  upon  it.  There  are 
many  other  facts  which  prove  that  the  surface  of  the  earth 
is  rising  in  some  places  and  sinking  in  others,  but  so  slowly 
that  we  do  not  perceive  it.  Slow  upward  and  downward 


PHYSICAL  CHANGES  OF  THE  OUTER  SHELL    65 

movements  of  this  sort  may  be  included  under  the  term 
warping. 

Local  crumpling  of  the  shell.  —  Before  they  were  consoli- 
dated, the  stratified  rocks  were  merely  layers  of  sediment 
which  had  been  deposited  in  a  horizontal  or  gently  inclined 
attitude.  In  many  places,  however,  we  now  find  them 
crumpled  and  folded  (Fig.  45).  The  folds  in  any  one  area 
are  usually  parallel  to  each  other,  and  are  arranged  in  long, 
narrow  bands.  Such  folds  have  evidently  been  produced 
by  compression  from  the  sides,  the  part  between  having 
wrinkled,  just  as  flat-lying  sheets  of  paper  will  wrinkle  if 
compressed  horizontally. 

Both  the  vertical  movements  mentioned  above  and  these 
lateral  movements  change  the  surface  features  of  the  earth. 
The  former  produce  plateaus,  plains,  and  broad  depressions, 
while  the  latter  make  mountain  ridges  with  troughs  between.1 

Causes  of  crustal  movements.  —  What  are  the  causes  of 
these  movements?  This  question  cannot  now  be  answered 
satisfactorily.  The  fact  that  these  movements  take  place  is 
undeniable,  but  the  causes  of  them  are  not  yet  fully  under- 
stood.2 

EFFECTS  OF  MOVEMENTS 

Having  now  in  mind  the  general  nature  of  these  slow  move- 
ments within  the  crust,  we  are  in  a  position  to  study  the  effects 
which  they  produce  in  the  rocks.  These  effects  may  be 
grouped  as  folds  on  the  one  hand  and  fractures  on  the  other. 

Fracturing  and  folding  of  rocks  compared. — Rocks  in  gen- 
eral are  brittle  substances.  If  quickly  bent  or  squeezed, 
they  will  break.  If,  however,  the  pressure  is  applied  very 
slowly,  and  especially  if  the  layer  is  kept  heavily  weighted 

1  Mountains,  plateaus,  plains,  troughs,  and  basins  are  formed,  not  only 
by  body  movements,  but  in  a  variety  of  other  ways,  which  are  discussed  in 
later  Chapters. 

2  The  theories  relating  to  crustal  movements  are  discussed  at  some  length 
in  larger  textbooks,  such  as  Chamberlin  and  Salisbury's  Geology,  Vol.  I, 
2d  ed.,  Chap.  IX. 

B.  &  B.  GEOL. — 5 


66  PHYSICAL  GEOLOGY 

clown  by  thousands  of  feet  of  rock  lying  upon  it,  a  bend  may 
result  instead  of  a  break.  Since  both  of  these  conditions 
exist  in  the  crust,  we  actually  find  the  rocks  broken  in  some 
places  and  bent  or  folded  in  others.  In  fact,  different  kinds  of 
rock  may  show  both  types  of  structure  in  the  same  place,  — 
the  stronger  rocks  being  broken,  while  the  weaker  are  folded. 

FOLDS 

Kinds  of  folds.  —  On  examining  the  layers  (or  strata)  of 
rock  over  a  large  area  we  may  find  them  flat  in  one  place, 
wavy  or  rolling  in  another,  and  intricately  twisted  and  crum- 
pled in  a  third,  with  all  gradations  between.  Thus  we  may 
describe  folding  in  general  as  simple  or  complex;  as  mild  or 
intense. 

The  individual  folds  may  be  classified  from  a  variety  of 
points  of  view.  Simplest  of  all  would  be  a  grouping  accord- 
ing to  their  attitude.  Thus,  all  folds  are  either  down  folds 
(synclines),  up  folds  (anticlines),  or  stepfolds  (monoclines). 
Usually  anticlines  and  synclines  are  combined  in  a  series  of 
undulations,  the  former  making  the  crests  and  the  latter  the 
troughs  of  the  waves. 


FIG.  46.  —  Gently  folded  sedimentary  rocks  in  the  central  part  of  the  Appa- 
lachian Moun tains.     (U.S.  Geol.  Surv.) 

Before  going  further  into  the  consideration  of  folds  we  may 
stop  to  examine  the  parts  of  a  single  simple  fold:  Each 
consists  of  two  limbs,  rising  to  a  crest  in  the  anticline  and 
descending  to  a  trough  in  the  syncline.  The  inclination  of  the 
limb  of  a  fold  is  called  the  dip.  In  field  studies  the  angle 
and  the  direction  of  the  dip  are  of  much  importance.  The  dip 
is  always  measured  downward  and  from  a  horizontal  plane. 
Thus  a  limb  having  a  dip  of  5°  would  be  nearly  level,  while 
one  with  a  dip  of  90°  would  be  vertical. 


PHYSICAL  CHANGES  OF  THE  OUTER  SHELL 


67 


When  many  anticlines  or  synclines  are  compared,  it  is 
found  that  they  present  a  wide  variety  of  forms.  Thus  there 
are  low,  broad  folds  (Fig.  46),  sharp  folds  (Fig.  47),  tightly 


FIG.  48.  —  Overturned  folds. 


FIG.  47. —  Closely  folded   strata  in  the  southern  part  of  the  Appalachian 
Mountains.     (U.S.  Geol.  Surv.) 

compressed  folds,  and  even  inclined  or  overturned  folds  (Fig. 
48).  A  layer  of  rocks  may  be  bent  into  any  of  these  forms 
according  to  the  condi- 
tions under  which  the 
pressure  was  applied. 

Competent  and  in- 
competent folds.  —  It  is 
often  advantageous  to 
classify  folds  according  to  their  competency.  In  order  to  form 
an  anticline  a  layer  must  have  a  certain  amount  of  strength. 
This  will  be  readily  apprehended  if  we  imagine  several  layers 
of  loose  sand  to  be  compressed  on  each  side,  —  they  would  be 
mashed  without  definite  folding.  As  compared  with  loose 
sand  or  mud,  we  can  well  understand  that  firm  beds  of  sand- 
stone or  limestone  would  be  likely  to  buckle  up  in  the  form  of 
folds.  Beds  which  are  strong  enough  to  hold  themselves  up 
in  arches  have  been  called  competent  strata,  while  materials 
which  may  be  squeezed  and  crushed  together  are  incompetent. 

In  considering  competency,  however,  it  is  necessary  to 
take  into  account  something  more  than  the  character  of  the 
rock.  Sheets  of  paper  lying  free  upon  the  table,  when  com- 
pressed sidewise,  will  arch  into  a  fold  and,  under  those  con- 
ditions, are  competent.  Nevertheless  if  several  books  are 
piled  upon  the  sheets,  the  latter  will  not  arch  when  com- 
pressed, but  will  merely  be  crumpled  into  many  little  twisted 
folds.  Similarly  any  layer  of  rock,  however  strong,  may  be 
so  weighted  down  by  overlying  beds  that  it  will  be  complexly 


68 


PHYSICAL  GEOLOGY 


crumpled  instead  of  being  folded  in  a  series  of  simple  waves. 

We  therefore  see  that  a  given  stratum  may  be  competent  if 

not  much  weighted,  but  incompetent  if  heavily  loaded  by 

reason  of  its  burial  deep  beneath  the  surface. 

In  accordance  with  these  facts  there  are  two  types  of  folds, 

one  characteristic  of  the  surface  layers  and  the  other  of  the 

great  depths.  When  a 
given  region  is  subjected 
to  compressive  horizontal 
forces,  the  layers  at  the  top 
may  arch  and  buckle  into 
open  folds;  while  those 
thousands  of  feet  beneath 
may  be  mashed  and  crum- 
pled into  many  little  broken 
parallel  crenulations  (Fig. 
49).  Between  these  there 
is,  of  course,  a  transition 
zone  wherein  the  weak 

FIG.  49. — Incompetent  folds  in  jasper  con-    rocks  Such  as  shale  will  be 
taming  streaks  of   ron  ore.    <U.S.Geol.    crushed,  while  the  stronger 

Compare  the  thickness  of  the  beds  near  limestones  and  Sandstones 
the  crests  and  troughs  of  the  folds  with  i  i      i        , 

the  thickness  along  the  limbs.    Why  the  may  be  merely  bent, 
difference  ?    Would  the  same  be  true  of         Folds        considered        in 

ground    plan. -Thus    far 

we  have  been  viewing  folds  in  cross  section.  In  order  to  see 
them  as  they  really  are  we  must  add  the  third  dimension  and 
regard  them  also  in  ground  plan. 

One  of  the  simplest  types  of  fold  is  the  dome,  in  which  the 
strata  dip  away  equally  in  all  directions  from  a  central  point. 
But  the  majority  of  folds  are  more  or  less  elongated  in  one 
direction.  If  unaltered  by  erosion,  they  would  form  long 
ridges  and  troughs,  gradually  decreasing  in  relief  toward  either 
end.  But  since  almost  all  the  folds  we  have  in  nature  have 
been  eroded,  —  many  of  them  having  been  completely  planed 
down,  —  it  is  better  to  consider  them  in  their  truncated  con- 


PHYSICAL  CHANGES  OF  THE  OUTER  SHELL 


69 


dition.  In  such  a  fold  we  find  a  definite  axis.  At  the  sides, 
the  edges  of  various  strata  are  nearly  parallel  to  this  axis. 
These  edges  are  outcrops,  and  their  shapes  depend  on  the 


FIG.  50. — Diagram  illustrating  the  difference  between  strike  and  outcrop. 
How  has  the  cutting  of  the  valleys  affected    (1)    the    outcrop,   and 
(2)  the  strike  ?    What  would  be  the  effect  if  the  beds  (1)  dipped  to  the 
right,  (2)  were  vertical  ? 

attitude  of  the  underlying  beds  and  upon  the  configuration  of 
the  hills  and  valleys  (Fig.  50).     The  strike  of  the  beds  is  the 


FIG.  51.  —  Stereogram  of  one  end  of  a  pitching  fold,  the  top  of  which  has 

been  cut  off. 


70  PHYSICAL  GEOLOGY 

line  made  where  the  inclined  stratum  is  cut  by  a  horizontal 
plane;  and  it  will  be  seen  that  it  is  always  at  right  angles  to 
the  direction  of  the  dip  of  the  same  beds. 

Obviously  a  fold  cannot  be  indefinitely  long.  It  dies 
out  at  either  end,  or  in  other  words,  the  axis  itself  is  gently 
arched  and  slants  downwards  in  opposite  directions  from  the 
highest  point  of  the  fold  (Fig.  51).  This  slant  of  the  axis  is 
called  the  pitch  of  the  fold  and  is  to  be  carefully  distinguished 
from  the  dip  of  the  beds.  In  a  circular  dome  the  angle  of 
pitch  is  the  same  as  that  of  the  dip  of  the  strata,  but  in  all 
other  folds  it  is  less,  except  along  the  axis.  Among  synclines 
the  features  are  much  the  same  except  that  the  pitch  and  dip 
are  directed  inward  toward  the  bottom  of  the  trough  and  the 
order  of  succession  of  the  strata,  as  they  outcrop  on  the  sur- 
face, is  reversed. 

FRACTURES 

Joints.  —  The  uneven  risings  and  sinkings  of  the  crust 
tend  to  crack  the  brittle  strata  in  every  direction.  Certain 
rocks,  such  as  drying  mud  and  cooling  lava,  crack  also  be- 
cause of  contraction.  Every  quarry  and  outcrop  shows 
parallel  systems  of  cracks,  usually  upright  where  the  rocks 
have  not  been  folded,  but  in  many  other  places  slanting. 
These  cracks  are  known  as  joints  (Fig.  52),  because  of  their 
rude  resemblance  in  some  instances  to  the  joints  between  the 
blocks  in  a  stone  wall. 

Fissility.  —  When  the  cracks  are  closely  spaced  and  parallel 
to  each  other,  the  rock  breaks  readily  into  plates.  Slates 
and  other  rocks  in  which  such  fracturing  prevails  are  said  to  be 
fissile  (Fig.  53) .  Almost  any  rock  may  be  either  fissile  or  j  ointed, 
or  both,  according  to  the  conditions  to  which  it  has  been  sub- 
jected. These  conditions  will  be  dealt  with  in  later  pages. 

Normal  faults.  —  Cracks  thus  divide  the  rocks  into  a 
multitude  of  blocks  of  various  sizes.  These  blocks  are  some- 
times tilted,  shifted,  or  let  down,  so  that  the  ends  of  the  broken 
strata  no  longer  match.  Such  dislocations  are  called  faults. 


PHYSICAL  CHANGES  OF  THE  OUTER  SHELL 


71 


Individual  faults  differ  widely  among  themselves,  and  to 
classify  or  interpret  the  many  kinds  is  a  matter  of  much 
complexity.  It  has  been  customary,  however,  to  group  the 
majority  of  them  in  two  divisions :  (1)  normal  faults,  and 
(2)  reversed  faults. 


FIG.  52.  —  Three  systems  of  joints  in  a  hard,  brittle  rock.     (Weidman.) 

Normal  faults  are  often  produced  by  warping ;  they  imply 
a  stretching  of  the  outer  part  of  the  crust  (Fig.  54).  The 
fault  planes  are  almost  invariably  steeply  inclined  or  vertical. 
The  moved  block  may  have  slipped  either  vertically,  diago- 
nally, or  horizontally.  It  is  often  possible  to  ascertain  the 
direction  of  this  slipping  by  means  of  the  polished  grooves, 
called  slickensides,  which  are  produced  by  the  grinding  of 
the  one  mass  of  rock  over  the  other.  The  vertical  distance 
between  the  broken  ends  of  a  given  stratum  measures  the 
amount  of  displacement  and  is  called  the  throw  of  the  fault 
(Fig.  55).  The  two  sides  are  designated  as  the  upthrow  and 


72 


PHYSICAL  GEOLOGY 


downthrow  sides,  and  it  is  important  to  note  that  in  normal 
faults  the  plane  of  slipping  slants  down  away  from  the  up- 
throw side. 


FIG.  53.  — Fissility  in  highly  tilted  beds  of  slate.     (Gilbert,  U.S.  Geol.  Surv.) 

Reversed  faults.  —  Where  the  rocks  have  been  compressed 
instead  of  stretched,  the  strata  may  be  broken  as  well  as 
folded.  Thus  the  second  type  of  faults  is  produced.  In  such 
cases  the  lower  and  therefore  older  rocks  are  shoved  up  and 
over  the  higher  and  younger.  This  is  the  reverse  of  the  con- 
dition in  a  normal  fault,  hence  the  name.  Some  reversed 


PHYSICAL  CHANGES  OF  THE   OUTER  SHELL         73 


faults  or  overthrusts  are  known  to  have  been  caused  by  the 
excessive  overturning  of  a  fold  (Fig.  56) ;  but  in  other  cases 


FIG.  54.  —  Diagram  of  normal  faults  in  a  segment  of  the  earth's  crust. 

the  rocks  have  merely  been  sliced  through  by  a  diagonal 
fracture  (Fig.  57). 

Reversed  fault  planes  are  usually  not  steeply  inclined. 
Some,  indeed,  are  almost  horizon- 
tal. In  the  mountains  of  North 
Carolina  great  masses  of  the  older 
rocks  have  been  thrust  along 
gently  inclired  fractures  for  dis- 
tances as  great  as  15  miles. 

Earthquakes.  -  -  The  disloca- 
tions which  result  in  faults,  es- 
pecially normal  faults,  are  often 
felt  at  the  surface  as  earthquake 
shocks.  Minute  slippings  in  the  rocks  give  rise  to  mere  tre- 
mors, which,  although  of  common  occurrence,  are  often 
imperceptible  to  our  senses.  Their  existence  is  detected  by 


FIG.  55.  —  Block  diagram  of  a 
normal  fault. 


FIG.  56. 


•An  overturned  fold  passing  into  a  reversed  fault. 
(After  Heim.) 


means  of  a  delicate  instrument  known  as  the  seismograph. 
Greater  ruptures  of  the  crust  generate  more  violent  shocks 
which  often  dislodge  huge  masses  of  loose  rock  from  moun- 


74 


PHYSICAL  GEOLOGY 


tain  slopes,  and,  where  they  affect  cities,  may  become  highly 
destructive  to  human  life  and  property.  The  Alaskan  earth- 
quake of  1899  resulted  from  a  sudden  displacement  of  more 


FIG.  57.  — Overthrusts  in  the  Highlands  of  Scotland.     (H.M.  Geol.  Surv.) 

than  40  feet;  and  the  San  Francisco  disaster  in  1906  was 
caused  by  a  horizontal  displacement  of  5  to  20  feet  (Fig.  58), 
which  took  place  along  a  line  many  miles  in  length.  Other 


FIG.  58.  —  Displacement  of  a  road  where  crossed  by  a  horizontal  fault. 
(Jones.)  The  San  Francisco  earthquake  of  1906  was  a  result  of  the  fault- 
ing. 

subterranean  shocks,  such  as  those  attending  volcanic  erup- 
tions, may  likewise  produce  violent  earthquakes. 

The  destructiveness  of  earthquakes  is  due  to  the  suddenness  of 
the  shock.  Thus  a  sharp  blow  struck  on  the  side  of  a  table  will 
cause  very  little  motion  in  the  table  itself,  but  it  may  be  sufficient 


PHYSICAL  CHANGES  OF  THE  OUTER  SHELL    75 

to  overthrow  completely  any  loose  objects  upon  the  table.  When 
an  earthquake  disturbs  the  sea  bottom  a  series  of  waves  is  set  in 
motion,  as  in  a  pan  of  water  sharply  tapped  on  the  side.  These 
earthquake  waves  (often  wrongly  called  "tidal"  waves)  rush  upon 
the  shore  and,  as  in  the  Sicilian  earthquake  of  1908,  may  wash 
away  houses  with  all  their  occupants,  and  may  even  dash  large  ships 
high  upon  the  beach. 

Slow  growth  of  faults.  —  Almost  all  large  faults,  whether 
of  the  normal  or  the  reversed  type,  have  probably  grown 
through  a  series  of  small  slips  separated  by  years  or  centuries 
in  which  no  movement  occurred.  Along  the  great  fault  at 
the  east  base  of  the  Sierra  Nevada  in  California  (Fig.  457), 
the  vertical  displacement  now  amounts  to  several  miles ;  but 
the  last  important  movement  along  this  fracture  occurred  as 
long  ago  as  1872,  and  at  that  time  the  dislocation  was  in- 
creased by  only  25  feet.  If  so  great  a  fault  were  to  be  made 
all  at  once,  the  shock  would  probably  wreck  every  building 
within  hundreds  of  miles.  Most  faults  grow  so  slowly  that 
the  scarp  (cliff)  on  the  upthrown  side  meanwhile  suffers  much 
from  the  work  of  weather  and  running  water  and  comes  to  be 
furrowed  by  many  ravines. 

UNCONFORMITIES 

In  many  places  the  older  beds  of  rock  are  folded  or  faulted 
or  are  cut  by  intrusive  bodies,  whereas  the  younger  beds  are 
undisturbed.  Thus  in  Figure  59  the  upper  layers  do  not  con- 
form in  structure  to  those  beneath.  The  lower  strata  were 
doubtless  nearly  level  when  first  deposited.  If  so,  they  have 
since  been  tilted  in  connection  with  folding  movements ;  but 
the  tops  of  the  folds  were  worn  off  before  the  sediments  which 
formed  the  upper  strata  were  laid  down.  The  two  sets  of 
layers  are  therefore  said  to  be  unconformable  and  the  contact 
is  an  unconformity. 

Not  all  unconformities  show  such  a  discordance  of  bedding. 
A  bed  of  sandstone  deposited  on  the  surface  of  a  planed-down 
mass  of  granite  shows  just  as  clearly  that  the  surface  was 


76  PHYSICAL  GEOLOGY 

deeply  eroded  before  the  sand  was  spread  over  it  (Why?). 
Likewise  an  irregular  weathered  surface  between  two  parallel 
beds  indicates  that  deposition  was  interrupted  after  the  first 
layer  was  deposited  and  that  erosive  processes  carved  the 


FIG.  59. — An  unconformity  in  Wyoming.     (Fisher,  U.S.  Geol.  Surv.) 

irregular  surface  before  deposition  was  resumed  in  forming  the 
upper  layers.  Although  not  so  conspicuous,  these  also  are 
cases  of  unconformity. 

It  is  obvious  that  unconformities  give  important  evidence 
of  changes  that  have  taken  place  in  previous  ages.  Their 
significance  is  further  discussed  in  later  chapters. 

VULCANISM 

Rise  of  lava  through  the  crust.  —  The  rise  of  molten 
rock  from  unknown  depths  into  the  outer  crust  may  be  ob- 
served and  is  proved  also  by  many  other  facts.  How  it 
becomes  molten,  how  far  down  it  originates,  why  it  rises  to  the 
surface,  and  how  it  makes  its  way,  are  difficult  problems,  none 
of  which  has  thus  far  been  satisfactorily  solved.1 

1  The  many  theories  which  have  been  suggested  to  answer  these  ques- 
tions are  discussed  in  some  of  the  larger  textbooks  of  Geology. 


PHYSICAL  CHANGES  OF  THE   OUTER  SHELL         77 

As  noted  in  the  preceding  Chapter,  lavas  which  have  suc- 
ceeded in  reaching  the  known  outer  part  of  the  lithosphere 
produce  various  structures  and  effects.  They  may  solidify 
beneath  the  surface  in  bodies  of  vast  size;  they  may  bulge 
the  overlying  rocks  in  blister-like  form ;  they  may  fill  cracks 
and  bedding  planes;  and  they  may  even  reach  the  surface, 
there  to  be  poured  out  as  lava  flows  or  be  blown  into  dust 
and  cinders.  Doubtless  that  part  which  solidifies  in  the  form 
of  batholiths,  stocks,  and  laccoliths  far  exceeds  the  portion 
which  has  been  built  into  surface  plateaus  and  the  familiar 
volcanic  cones.  All  of  these  are  effects  of  one  great  process, 
vulcanism. 

The  building  of  characteristic  structures  and  the  formation 
of  igneous  rocks  is  not  the  only  conspicuous  effect  of  vulcan- 
ism. The  lavas  may  bake  the  rocks  which  they  penetrate. 
The  hot  gases  and  solutions,  which  most  bodies  of  lava  emit 
as  they  cool  and  crystallize,  spread  out  through  cracks  and 
pores  in  the  surrounding  rocks  and  deposit  quantities  of 
minerals  which  they  originally  held  in  solution.  By  these 
means  the  country  rock  may  be  considerably  altered. 

ZONES  OF  FRACTURE  AND  FLOW  AGE 

As  compared  with  other  things  familiar  to  us,  rock  is  one  of 
the  hardest  and  strongest  materials.  Nevertheless,  as  we 
have  already  intimated  in  describing  folds,  there  is  a  limit  to 
its  strength. 

At  the  surface  the  rock  lies  under  the  weight  of  the 
atmosphere  only  (about  15  pounds  per  square  inch).  At 
a  depth  of  one  mile  there  is  added  to  this  the  weight  of 
a  column  of  rock  one  mile  high,  or  about  6300  pounds  per 
square  inch.  This  is  enough  to  crush  the  softer  limestones 
and  sandstones.  At  a  depth  of  6  miles  or  more  the  pressure 
is  so  much  greater  that  even  the  strongest  rocks  cannot  resist 
it.  Each  grain  is  compressed  into  the  smallest  space  it  will 
occupy,  and  any  cavities  or  pores  existing  in  rocks  at  that 


78  PHYSICAL  GEOLOGY 

depth  must  be  tightly  closed.  Into  this  deep  zone,  then, 
water  cannot  readily  penetrate,  because  there  are  no  cracks 
and  pores  to  afford  it  passage.  If  crustal  movements  take 
place,  these  deeply  buried  rocks  cannot  break,  but  the  over- 
whelming pressure  forces  them  to  yield  or  flow  like  a  plastic 
mass  of  putty.  This  deep  zone  has  therefore  been  called  the 
zone  of  flowage. 

At  and  near  the  surface  almost  any  rock  is  strong  enough 
to  maintain  open  fissures,  cavities,  and  pores.  Crustal  move- 
ments there  tend  to  crack  the  rocks,  the  pressure  being  in- 
sufficient to  mold  them.  The  upper  zone  is  therefore  called 
the  zone  of  fracture. 

Between  these  two  zones  is  a  transition  zone  in  which  the 
weaker  rocks,  such  as  shale,  yield  to  the  pressure;  while 
strong  rocks,  like  quartzite  and  granite,  remain  rigid  and  may 
support  open  fissures. 

How  ROCKS  ARE  ALTERED 

Like  most  other  substances,  rocks  may  be  radically  changed 
by  subjecting  them  to  great  pressure,  heat,  and  the  solvent 
action  of  water.  Some  of  these  alterations  result  in  the  decay 
of  the  rocks,  some  in  further  hardening,  and  some  in  a  reor- 
ganization of  the  minerals  of  which  the  rock  is  composed. 
These  changes  are  in  turn  the  cause  of  differences  in  the  color, 
strength,  structure,  and  texture  of  the  rock.  A  red  sandstone 
may  be  changed  into  a  hard,  black  quartzite ;  a  massive  basalt 
may  become  a  slaty  green  schist ;  and  a  dense  limestone  may 
be  altered  to  a  coarse-grained  marble. 

Alterations  in  the  zone  of  fracture.  —  The  rain  water 
which  sinks  into  the  soil  accumulates  in  the  pores  and  cracks 
in  the  rocks  beneath.  Except  near  the  surface  it  completely 
saturates  the  rocks,  and  the  body  of  water  thus  formed  is  the 
source  of  our  wells  and  springs.  At  first  the  water  goes 
chiefly  downward ;  but  soon  it  begins  to  travel  devious  paths 
through  cracks  and  porous  layers,  many  of  which  lead  it  in 


PHYSICAL  CHANGES  OF  THE  OUTER  SHELL    79 


horizontal  or  diagonal  directions.  Afterward  a  part  of  it 
may  issue  at  lower  points  in  springs.  To  this  pervasive  cir- 
culation of  water  are  due  many  of  the  most  important  changes 
which  rocks  undergo. 

In  its  early  downward  course,  percolating  water  dissolves 
out  of  the  rocks  the  more  soluble  materials.  This  renders  the 
rocks  more  porous  and  decayed,  until  they  crumble,  leaving 
the  less  soluble  sand  and  clay  to  form  the  soil.  These  in  turn 
are  likely  to  be  carried  off  by  winds,  streams,  and  other 
agencies  and  deposited  as  beds  of  sand  and  mud,  which  may 
eventually  become  sandstone  and  shale. 

In  the  course  of  its  descending  journey  the  water  thus  be- 
comes saturated  with  various  mineral  materials.  When  in 
this  state  a  slight  change  in 
the  temperature,  or  other 
surrounding  conditions,  may 
be  sufficient  to  cause  the 
deposition  of  some  of  the 
dissolved  substance.  Thus 
quartz  may  crystallize  out 
of  a  solution  which  is  slowly 
seeping  through  a  porous 
bed  of  sandstone  and  may, 
by  filling  up  the  pores,  pro- 
duce a  firmly  cemented 
quartzite  (Fig.  60).  Only  a 
part  of  the  mineral  matter 
taken  into  solution  during 
the  decay  of  the  rocks  is 
used  up  in  this  way;  some  of  it  is  carried  out  through  springs, 
joins  rivers,  and  is  finally  poured  into  the  sea. 


FIG.  60.  —  Quartzite  as  it  appears 
under  the  microscope.  The  individ- 
ual sand  grains  may  still  be  identi- 
fied by  their  rounded  outlines,  but 
the  interspaces  are  completely  filled 
with  quartz. 


Minerals  often  crystallize  upon  the  walls  of  a  fissure  through 
which  a  water  solution  is  rising,  and  by  this  process  the  crack  may  be 
completely  filled.  The  result  is  a  mineral  vein.  The  commonest 
vein  minerals  are  quartz  and  calcite  ;  but  occasionally  rare  and 
valuable  minerals,  such  as  gold,  silver,  compounds  of  lead,  zinc,  and 


80 


PHYSICAL  GEOLOGY 


Removed  \    \  by  Erosion 

\         I 


copper,  which  happen  to  be  dissolved  by  the  waters  elsewhere,  are 
deposited  in  veins.  A  fissure  with  a  valuable  metalliferous  filling 
of  this  sort  is  an  ore  vein  (Fig.  61). 

Thus  it  appears  that  the  zone  of  fracture  may  be  divided 
into  two  fairly  distinct  belts  according  to  the  nature  of  the 

changes  which  affect  the 
rocks.  Near  the  surface, 
and  especially  above  the  top 
of  the  body  of  underground 
water,  the  rocks  are  parti- 
ally dissolved  and  tend  to 
decay.  This  is  the  belt  of 
weathering.  At  greater 
depths,  and  chiefly  in  the 
region  where  the  rocks  are 
saturated  with  water,  pores 
and  cracks  are  gradually 
filled  and  the  whole  mass 
becomes  cemented.  This  is 
the  explanation  of  the  sin- 
gular fact  that  many  deep 
mines  are  dry ;  there  are  no 
longer  any  passageways 
through  which  water  may 
flow.  In  this  belt  of  cementa- 
tion, marked  chemical 
changes  in  the  minerals  themselves  are  in  progress,  resulting 
in  the  formation  of  new  minerals  out  of  old  ones.  Thus  gar- 
net may  change  to  chlorite,  and  augite  may  become  horn- 
blende.1 

Alterations  in  the  zone  of  flowage.  —  With  the  pressure  so 
great  as  it  must  be  beneath  5  to  6  miles  of  solid  rock,  it 

1  When  a  mineral  is  thus  changed  gradually  into  one  of  different  com- 
position, without  altering  the  original  form,  the  result  is  a  pseudomorph. 
Petrified  wood,  although  not  originally  a  mineral,  is  a  pseudomorph  in  this 
sense. 


-  Unenriched  Vein 


FIG.  61.  —  Diagram  of  a  copper  vein. 
The  entire  vein  was  once  rather  lean 
like  the  lower  part,  but  as  the  surface 
was  lowered  by  erosion  much  of  the 
copper  ore  removed  soaked  down  into 
the  vein  below  and  there  formed  a 
particularly  rich  deposit  (solid  black). 


PHYSICAL  CHANGES  OF  THE  OUTER  SHELL     81 

will  surprise  no  one  to  learn  that  the  mineral  grains  are  there 
crushed  into  minute  fragments  and  packed  closely  together. 
Firm  quartzites  may  be  mashed  or  granulated  until  all  trace 
of  the  original  sand  grains  is  lost;  and  in  coarse  gran- 
ites scarcely  a  crystal  may  be  left  in  its  original  size  or 
shape. 

But  other  influences  besides  pressure  are  at  work.  Many 
things  indicate  that  the  earth  is  hot  within.  The  deepest 
mines  are  uncomfortably  warm  even  in  midwinter.  Vol- 
canoes and  hot  springs  scattered  widely  over  the  surface  tell 
of  much  greater  heat  beneath.  In  mines  and  borings 
the  temperature  rises  on  the  average  about  1°  for  every 
60  to  90  feet  of  descent.  If  this  rate  holds  good,  the 
rocks  in  the  zone  of  flowage  should  be  hotter  than  350°  C. 
(=  662°  F.). 

Water  is  another  factor.  Although  it  is  true  that  the 
circulation  of  water  in  this  deep  zone  is  greatly  impeded  by 
the  general  lack  of  cracks  and  pores,  yet  water  is  every- 
where present;  and,  at  such  high  temperatures,  it  may 
be  in  the  form  of  steam,  notwithstanding  the  great  pres- 
sure.1 

This  superheated  water  or  steam  is  a  powerful  solvent. 
In  it  many  minerals  dissolve,  and  they  may  crystallize  out 
again  in  new  forms,  which  are  better  adapted  to  the  pressure. 
The  new  minerals  produced  are  usually-  heavier  and  denser 
than  those  which  were  dissolved,  for  in  a  dense  mineral  the 
same  amount  of  material  occupies  less  space,  a  change  de- 
manded by  the  overpowering  pressure. 

As  the  new  minerals  crystallize,  it  is  easier  for  them  to  grow 
at  right  angles  to  the  greatest  pressure  than  directly  against 
it.  Thus  all  the  crystals  generally  come  to  be  elongated  in  the 
same  direction,  and  the  rock  takes  on  a  banded  or  streaky 

1  Water  passes  into  steam  at  100°  C.  (212°  F.)  under  the  ordinary  pres- 
sure of  the  air  at  sea  level.  Under  twice  that  pressure  it  boils  at  120°  C. 
(248°  F.)  ;  and  at  ten  times  the  air  pressure,  at  180°  C.  (356°  F.).  Above 
356°  C.  (673°  F.)  steam  cannot  be  forced  into  the  liquid  state  by  any  pres- 
sure that  has  ever  been  applied.  This  is  called  its  critical  temperature. 
B.  &  B.  GEOL. 6 


82 


PHYSICAL  GEOLOGY 


are    characteristic    of 
phosed  igneous  rocks. 


unmetamor- 


aspect  (Figs.  62  and  63). 
Where  the  cleavage  planes  of 
the  minerals  are  parallel  to 
the  crystals,  the  whole  rock 
may  split  readily  along  them, 
and  the  result  is  schist,  if  the 
crystals  are  large  enough  to 
see,  or  slate,  if  the  rock  is 
dense. 

The   result   of  these   pro- 
cesses is  a  more  or  less  com- 
FIG.  62.  -Gabbro  as  it  appears  under  Plete  change  in  the  character 

the  microscope.    The  white  bodies    of    the    rock.      Although     the 
are  feldspar,  the  shaded  bodies  au-  •,  •  -,         •     i  •  -, 

gite,  and  the  black  spots  magnetite,  mashing  and  grinding  down 

The  irregular  forms  of  the  crystals    of  the    particles    tend  to  prO- 

duce  a  loose,  fine-grained 
rock,  the  crystallizing  of  the 

material,  on  the  other  hand,  produces  a  firm,  coarser-grained 

rock,  which  also  is  usually  banded  or  cleavable. 
Rocks  which  are  invaded 

by  hot  lavas  from  below 

are    subjected    locally    to 

conditions  not  unlike  those 

of   the   zone    of    flowage. 

Under  the  influence  of  the 

high   temperature   and  of 

the  hot  solutions  and  gases 

which  emanate  from  the 

lava,  many  rocks  recrys- 

tallize  and  undergo  other 

radical  changes.     A  lime- 
stone may  become  a  hard 

Crystalline     rock      charged  FlG*  63'~ Schistas  it  appears  under  the 

.,,  „  microscope.      The    black,    white,     and 

With     Crystals     of     garnet,       shaded  bodies  are  mineral  particles  of 

hornblende,  and  other  new     three  different  kinds. 

i          a,     !  What  would  be  the  direction  of  cleav- 

mmerals.    Shale   may   be     age  in  this  rock? 


PHYSICAL  CHANGES  OF  THE  OUTER  SHELL     83 

baked  into  a  dense  flinty  rock  in  which  the  microscope  shows 
that  an  abundance  of  little  crystals  have  formed.  Since  pres- 
sure may  be  much  less  effective  here  than  in  the  deep  zone, 
the  rocks  altered  along  volcanic  contacts  do  not  generally  pos- 
sess the  cleavage  and  banded  structure  which  are  the  results 
of  recrystallization  under  great  compression. 

The  metamorphic  cycle.  —  When  the  changes  of  the  two 
zones  are  put  together,  it  is  seen  that  they  form  part  of  a 
nearly  complete  cycle  of  alterations.  By  way  of  illustration 
let  us  take  an  igneous  rock,  such  as  granite.  In  the  belt  of 
weathering  it  decays ;  the  complex  minerals  such  as  feldspar 
and  mica  are  changed  into  simpler  chemical  compounds,  and 
of  these  some  are  dissolved,  while  the  remainder,  with  the 
unchanged  quartz,  forms  soil.  When  carried  away  and 
assorted  by  water,  this  residue  makes  beds  of  sand  and  clay, 
and  at  the  same  time  some  of  the  dissolved  substances  are 
deposited  as  limy  ooze.  Thus  far  the  process  is  destructive. 

Gradually  buried  by  more  sediments,  the  sand,  clay,  and 
ooze  come  to  lie  in  the  belt  of  cementation  and  later  in  the 
zone  of  flowage.  In  the  former  they  are  consolidated,  through 
the  processes  of  cementing  and  crystallizing,  into  firm  sand- 
stone, shale,  and  limestone ;  in  the  latter,  the  simple  minerals 
of  which  they  are  composed  are  mashed,  recrystallized,  and 
combined  in  the  form  of  more  complex  minerals  which  to- 
gether form  solid  crystalline  rocks,  in  some  respects  not 
unlike  the  original  granite.  Here  the  change  is  constructive. 
If  by  still  deeper  burial  the  rocks  could  be  heated  to  the 
melting  point,  they  might  actually  be  made  over  again  into 
igneous  rocks  and  thus  complete  the  cycle.  Whether  this 
has  occurred,  however,  is  doubtful. 

QUESTIONS 

1.  Figures  64  to  67  are  maps  on  which  the  outcrops  of  the  strata 
are  shown  as  bands.  The  dip  and  strike  are  indicated  by  the  usual 
sign.  The  beds  are  numbered  in  the  order  of  their  age,  "  1 "  being 
the  oldest. 


84 


PHYSICAL  GEOLOGY 


FIG.  64. 


FIG.  65. 


FIG.  66. 


FIG.  67. 


What  structure  in  the  rocks  beneath  is  indicated  by  the  obser- 
vations in  each  case  ? 

2.  Show  by  sketch  maps  the  surface  of  the  outcrops  correspond- 
ing to  the  beds  shown  in  the  cross  sections  (Figs.  68  to  71).  Num- 
ber the  strata  and  show  the  dips. 


FIG.  68. 


FIG.  69. 


FIG.  70. 


FIG.  71. 


3.  See  Figures  45,  46,  47,  290,  291,  and  414.     Are  the  folds  of 
the  competent  or  the  incompetent  type  ? 

4.  Show  by  sketch  maps  how  the  outcrops  may  be  changed  by 
faulting  along  the  lines  represented  in  Figures  72,  73,  and  74.     Let 
the  faults  be  normal  and  inclined  toward  the  right  in  each  case. 
Assume  that  the  fault  cliffs  are  cut  down  to  a  plain  surface  by 
erosion. 

5.  Explain  the  outcrops  shown  in  Figure  75  and  illustrate  by 
diagrams. 


PHYSICAL  CHANGES  OF   THE- OUTER  SHELL 


85 


FIG.  72. 


FIG.  73. 


6.  Why  are  reversed  faults  usually  associated  with  folds  ?     Why 
are  the  strikes  of  the  two  features  usually 

parallel  ? 

7.  Why  should  normal  faults  be  expected 
to  disappear  downward  ? 

8.  Why  should  earthquakes  be  more  de- 
structive to  buildings  situated  on  unconsoli- 
dated  clay  than  to  those  on  solid  rock  ? 

9.  If  of  two  springs  in  the  same  region, 
one  is  hot  and  the  other  cold,  which  may  be 
supposed  to  have  the  deeper  source? 

10.  What  phase  of  metamorphism  is  il- 
lustrated by  the  change  of  (1)  gravel  to  conglomerate,  (2)  chalk  to 
limestone,  (3)  peat  to  hard  coal,  (4)  basalt  to  clay,  (5)  mud  to  slate? 


FIG.  74. 


FIG.  75. 


REFERENCES 

CHAMBERLIN  and  SALISBURY  :   Geology,  Vol.  I.     (New  York,  1904.) 
GEIKIE,  A.  :    Text-book  of  Geology,  Vol.  I.     (London,  1903.) 
GEIKIE,  J.  :    Structural  and  Field  Geology.     (New  York,  1905.) 
JUKES-BROWNE  :    Handbook  of  Physical  Geology.     (London,  1892.) 
VAN  HISE  :   Principles  of  North  American  Pre-Cambrian  Geology,  in 

16th  Ann.  Kept.,  U.S.  Geol.  Surv.,  Pt.  I,  pp.  571-843. 
—  A  Treatise  on  Metamorphism;  Mono.  XLVII,  U.S.  Geol.  Surv. 
WILLIS  :    Mechanics  of  Appalachian  Structure,  in  13th  Ann.  Kept., 

U.S.  Geol.  Surv.,  Pt.  II,  pp.  211-282. 


CHAPTER  III 
THE   WORK   OF   THE   ATMOSPHERE 

THE  erosive  effects  of  the  atmosphere  upon  the  surface 
of  the  land 'in  general  are  less  important  than  those  of  running 
water,  because  water  is  much  heavier  (over  800  times),  and  its 
work  is  concentrated  for  the  most  part  along  definite  lines. 
Yet  the  atmosphere  is  an  important  geological  agent  because 
of  its  extreme  mobility,  and  because  two  of  its  constituents, 
carbon  dioxide  and  oxygen,  by  uniting  chemically  with  many 
rocks,  change  their  character  and  cause  them  to  decay.  The 
atmosphere  is  also  indirectly  of  great  importance.  This  is  evi- 
dent when  it  is  remembered  that  without  an  atmosphere  there 
would  be  no  life  upon  the  lands,  no  precipitation  of  rain  or 
snow,  and,  in  consequence  of  this,  no  work  by  streams  and 
glaciers. 

The  work  of  the  atmosphere  may  be  considered  under  two 
main  headings :  (1)  the  work  it  accomplishes  by  mechanical 
means,  and  (2)  that  accomplished  by  chemical  means. 

MECHANICAL  WORK 

The  mechanical  work  of  the  atmosphere  is  performed  largely 
by  the  wind,  and  consists  chiefly  in  transporting,  wearing, 
and  depositing  rock  materials.  The  atmosphere  also  in- 
fluences the  changes  in  rocks  that  are  produced  by  variations 
in  temperature. 

TRANSPORTATION 

Transportation  by  the  wind  is  likely  to  be  important  wher- 
ever dry  surfaces  of  fine  material  are  exposed  to  strong  winds. 
These  conditions  are  fulfilled  best  over  large  areas  in  desert 

86 


THE  WORK  OF  THE  ATMOSPHERE  87 

and  semiarid  regions.  When  it  is  noted  that  desert  regions 
cover  about  11,500,000  square  miles,  or  over  one  fifth  of  the 
land  of  the  world,  it  is  evident  that  the  areas  where  wind  work 
is  of  prime  importance  are  by  no  means  restricted.  It  is  not 
apparent,  furthermore,  that  desert  areas  are  relatively  more 
extensive  now  than  at  various  times  in  the  past,  so  that  the 
wind  has  been  a  very  important  agent  of  change  through  long 
ages.  Although  it  is  most  important  in  dry  regions,  the  work 
of  the  wind  is  by  no  means  confined  to  them.  Rather  is  it 
world  wide. 

Material  gets  into  the  air  in  many  ways.  It  is  picked  up 
by  ascending  air  currents,  given  out  by  chimneys,  stirred  up 
along  dusty  roads  by  animals  and  vehicles,  discharged  by 
volcanoes,  and  delivered  to  the  air  in  a  variety  of  other  ways. 
Once  in  the  air,  gravity  tends  to  pull  it  back  to  the  surface, 
but  its  fall  is  retarded  by  friction  with  the  atmosphere.  If 
the  material  is  fine  (dust),  the  surface  it  exposes  to  the  friction 
of  the  air  is  great  in  proportion  to  its  weight,  and  it  settles 
very  slowly.  Before  reaching  the  ground,  it  may  encounter 
ascending  currents  and  be  carried  up  with  them.  When  it 
falls  again,  it  may  meet  and  be  lifted  by  other  up-going  cur- 
rents, or  may  settle  to  the  ground.  Fine  dust  carried  up  to 
great  elevations  has  sometimes  remained  in  the  air  for  many 
weeks  at  a  time.  After  it  reaches  the  surface,  it  may  be 
moved  repeatedly  by  the  wind. 

The  amount  of  dust  in  the  air  at  one  time,  even  in  moist 
regions,  is  often  very  great.  In  addition  to  vast  numbers  of 
larger  dust  particles,  the  air  in  many  places  (e.g.  great  cities) 
contains  hundreds  of  thousands  of  invisible  dust  motes  per 
cubic  centimeter  (about  f  of  a  cubic  inch).  It  has  been 
estimated  that  in  violent  dust  storms  the  air  may  contain  as 
much  as  126,000  tons  of  dust  and  sand  per  cubic  mile. 

Dust  is  transported  great  distances  by  the  wind.  It  settles 
on  ship  deck  in  mid-ocean,  has  been  carried  from  volcanoes 
to  snow-capped  mountains  in  distant  parts  of  the  world, 
where  it  was  found  subsequently,  and  in  the  exceptional  case 


PHYSICAL  GEOLOGY 


of  the  eruption  of  Krakatoa  in  1883,  it  was  carried  repeatedly 
around  the  earth  in  diminishing  amount,  its  progress  being  re- 
corded by  the  brilliant  sunsets  which  it  occasioned.  It  has, 
indeed,  been  suggested  that  every  place  upon  the  surface  of 
the  earth  may  possibly  have  dust  brought  by  the  wind  from 
every  other  place  upon  the  land. 

Much  wind-transported  material  settles  upon  the  oceans. 
The  aggregate  effect  of  wind  transportation  is  therefore  to 
lower  the  lands  and  to  raise  the  ocean  floors,  and  this  has  been 
the  case  since  the  continents  and  oceans  were  formed.  Al- 
though the  result  is  insignificant  within  any  short  period,  it  is 
doubtless  important  in  the  long  ages. 

Fine  material  is  carried  higher  and  farther  by  the  wind 
than  coarse  material,  and  it  settles  upon  the  surface  more 
evenly,  rarely  forming  surface  fea- 
tures. Sand,  on  the  other  hand,  is 
rolled  and  dragged  along  the  ground, 
or  lifted  but  a  few  feet  above  it,  and 
therefore  encounters  many  obstacles, 
such  as  trees,  fences,  buildings,  and 
the  like,  about  which  it  may  lodge 
to  form  hills  or  mounds. 

ABRASION 


FIG.  76.  —  Telegraph  pole 
near  Palm  Springs  Sta- 
tion, southern  Califor- 
nia, deeply  cut  by  wind- 
driven  sand .  The  stones 
have  been  placed  about 
the  bottom  of  the  pole 
to  protect  it  so  far  as 
possible  from  the  sand. 
(Mendenhall,  U.S.  Geol. 
Surv.) 


How  accomplished.  —  Wind  of  itself 
can  do  little  or  nothing  in  the  way  of 
wearing  solid  rocks,  but  the  sand  par- 
ticles it  often  carries  serve  as  effective 
tools  which  cut  and  wear  the  surfaces 
against  which  they  are  driven.  The 
wearing  power  of  wind-blown  sand 
may  be  illustrated  in  various  ways. 


It  is  shown  by  the  artificial  sand 
blast,  a  process  in  which  glass  is  etched  by  sand  driven  against 
it  with  force.  The  glass  in  car  windows  has  been  destroyed 


THE  WORK  OF  THE  ATMOSPHERE 


89 


in  a  single  day  in  passing  through  severe  sand  storms.  In 
many  places  telegraph  poles  must  be  protected,  or  the  wind 
cuts  them  down  in  a  comparatively  short  time  (Fig.  76). 
Wind  wear  (abrasion),  like  wind  transportation,  is  most 
important  in  dry  regions, 
for  in  such  places  slopes 
are  often  bare  and  unpro- 
tected, and  the  winds,  fre- 
quently strong,  are  likely 
to  be  abundantly  supplied 
with  tools. 

Characteristics  of  wind- 
worn  surfaces.  —  The  de- 
tails of  wind-worn  surfaces 
depend  on  the  strength 
and  structure  of  the  rocks. 


FIG.  77.  —  Wind-worn  surface  in  Wyo- 
ming. The  protruding  masses  are  harder 
than  the  rock  which  surrounds  them. 

Why  are  these  rocks  not  etched  like 
those  of  Figure  78  ? 

Rocks  of  varying  strength  wear 
unequally  (Fig.  77),  and  often  in  the  case  of  stratified  rocks 
the  more  rapid  removal  of  the  weaker  layers  or  laminae 
leaves  the  stronger  ones  in  relief  (Fig.  78) .  Horizontal  beds 

in  deserts  may  be  eroded 
into  extensive  flat-topped 
elevations,  which  are  later 
cut  up  into  abrupt  conical 
hills,  and  finally  destroyed. 
Inclined  beds  (whether 
worn  by  wind  or  water) 
tend  to  develop  hills  hav- 
ing a  relatively  long  and 
gentle  slope  in  the  direction 
in  which  the  beds  dip,  and 
a  shorter  and  steeper  op- 
posite facing  slope.  Wind- 
abraded  elevations  generally  suffer  most  rapid  wear  near 
the  bottom,  in  spite  of  the  fact  that  the  wind  is  retarded 
by  friction  with  the  surface  of  the  ground  and  its  blows 
thereby  weakened,  for  many  more  tools  are  carried  in  the  low- 


FIG.  78.  —  Cross-bedded  sandstone 
etched  by  the  wind. 


90  PHYSICAL  GEOLOGY 

ermost  few  feet  of  air  than  in  the  more  swiftly  moving  cur- 
rents above.  As  a  result,  such  elevations  are  often  undercut, 
and  have  steep  and  even  overhanging  slopes.  Small,  isolated 
elevations  may  take  on  the  form  of  great  mushrooms.  If  the 
pedestal  is  worn  through,  the  larger  upper  part  falls,  and  may 
be  worn  away  in  turn.  Wind-worn  slopes  are  frequently 
characterized  also  by  the  absence  of  accumulations  of  angular 
fragments  (talus)  at  their  base  (Why?).  Hard,  compact  stones 
on  wind-swept  surfaces  are  sometimes  given  flat  and  highly 
polished  faces. 

Erosion  in  deserts.  —  The  final  result  of  erosion  in  an  in- 
terior desert  basin  is  the  formation  of  a  nearly  level  rock- 
floored  plain,  covered  more  or  less  generally  but  thinly  with 
hard,  gravelly  waste,  and  surmounted  here  and  there  by  eleva- 
tions representing  the  most  resistant  portions  of  the  beds  that 
have  been  worn  away.  In  the  earlier  stages  of  the  production 
of  such  a  plain,  when  the  slopes  of  the  basin  are  still  steep,  the 
work  of  intermittent  desert  streams  and  occasional  floods  is 
more  important  than  that  of  the  wind.  It  is  to  be  noted  that 
the  driest  regions  of  the  world  now  and  then  receive  sufficient 
rains  to  cause  floods.  In  the  later  stages,  however,  streams  be- 
come less  important  as  the  surface  becomes  more  even,  and 
the  wind  comes  to  play  the  leading  role.  Whirlwinds  lift  dust 
high  into  the  air,  where  it  is  taken  up  by  the  upper  currents 
and  carried  outside  the  desert  area.  Strong  winds  sweep 
sand  across  the  basin  floor,  and,  if  the  inclosing  slopes  are  not 
too  steep,  carry  it  over  the  rim  to  outside  regions.  By  the  re- 
moval of  its  waste  in  these  ways,  the  basin  is  slowly  lowered. 
During  the  process  the  wind  may  scour  out  depressions  where 
the  rocks  of  the  desert  floor  are  weak,  but  such  depressions 
cannot  become  deep,  because  of  the  inwash  of  waste  from  the 
surrounding  higher  ground  by  temporary  streams  born  of 
occasional  showers.  Thus  the  streams  serve  as  a  check  upon 
the  winds,  and,  as  indicated  above,  an  old-age  desert  plain  is 
nearly  level.  Wind-degraded  plains  of  interior  basins  differ 
from  normal  old-age  plains  developed  by  running  water 


THE   WORK  OF  THE  ATMOSPHERE  91 

in  regions  draining  to  the  sea,  in  that  the  latter  cannot  be  worn 
below  sea  level  (p.  139)  while  the  former  are  independent  of 
it.  The  two  are  alike  in  having  nearly  level  surfaces,  which 
are  independent  of  the  underlying  rock  structures.  Old 
desert  plains  with  rock  floors  are  said  to  cover  thousands  of 
square  miles  in  South  Africa. 

DEPOSITION 

Formation  of  dunes.  —  It  has  already  been  pointed  out 
(p.  88)  that  the  deposition  of  dust  by  the  wind  rarely  gives 
rise  to  topographic  features  of  importance,  while  that  of 
sand  frequently  does.  Elevations  of  wind-deposited  sand 
are  dunes.  Figure  79  shows  how  sand  begins  to  accumulate 


FIG.  79.  —  Sketch  from  photograph  showing  how  sand  accumulates  about 
an  obstacle  which  it  cannot  penetrate. 

about  an  obstacle  which  it  cannot  penetrate.  It  is  deposited 
on  both  the  windward  and  leeward  sides,  but  is  prevented 
from  resting  directly  against  the  obstacle  by  the  air  that  is 
reflected  from  it,  and  by  wind  eddies.  In  the  case  of  a  pene- 
trable obstacle,  such  as  a  hedge  or  open  fence,  the  sand  lodges 
chiefly  on  the  leeward  side  (Fig.  80).  Figure  81  shows  sand  be- 
ginning to  gather  in  and  about  obstructing  vegetation.  Once 
started,  a  dune  causes  the  lodgment  of  more  sand,  and  so 
occasions  its  own  growth. 

Distribution,  size,  and  shape  of  dunes.  —  Dunes  occur 
chiefly  along  sea  and  lake  shores,  along  sandy  valleys,  and  in 
deserts ;  in  a  word,  where  quantities  of  bare,  dry  sand  are  ex- 


92 


PHYSICAL  GEOLOGY 


posed  to  strong  winds.   They  range  in  height  from  a  foot  or  two 
up  to  300  or  400  feet,  and  in  rare  cases  even  more.     The  great 

majority  do  not  exceed  20  feet. 
In  cross  section  they  are  typi- 
cally as  shown  in  Figure  82. 
The  longer  and  gentler  side  faces 


FIG.  80.  —  Deposit  of  sand  along    FIG.   81.  —  Beginning   of   a  dune  on   the 
the  side  of  a  fence.  beach  of    Lake    Michigan,   near    Dune 

Park,  Ind.     Shows  how  sand   accumu- 
lates about  obstructing  vegetation. 

the  dominant  wind,  wh  le  the  shorter  and  steeper  side 
is  the  leeward  slope.  The  windward  slope  is  a  roadway  up 

which  sand  is  rolled  and  dragged 
to  the  crest,  behind  which  it  drops. 
FIG.  82. —Cross  section  of  a       Its    steepness    varies    with    the 

strength  of  the  wind,  and  the  size 

and  quantity  of  the  sand.  Strong  winds  are  able  to  move 
a  small  amount  of  fine  sand  up  relatively  steep  slopes ; 
weak  winds  burdened  with  much  coarse  sand  require  a  gentle 
grade.  The  leeward  slope  represents  the  angle  at  which  the 
sand  will  rest,  and  rarely  exceeds  24°  or  26°.  Dune  sand  is 
often  distinctly,  but  irregularly,  stratified  (Fig.  83).  Dunes 
vary  in  ground  plan  from  roundish  mounds  and  hills  to  elongate 
ridges.  Examples  of  various  types  of  dunes  are  shown  on  the 
topographic  maps  of  Plate  I. 

In  order  to  read  topographic  or  contour-line  maps,  it  is  neces- 
sary to  know  that  contours  are  lines  drawn  on  maps  to  express 


THE   WORK  OF  THE  ATMOSPHERE  93 

relief  (inequalities  of  surface),  and  that  any  given  line  runs  through 
points  of  the  same  elevation  above  sea  level.  This  will  be  under- 
stood readily  by  reference  to  Figures  84  and  85.  Figure  84  shows  a 
model  of  an  ideal  landscape  viewed  from  above,  on  which  lines 
have  been  drawn  connecting  places  of  equal  elevation.  In  Figure  85 
the  above  lines  are  shown  alone  ;  this  is  a  contour  map  of  the 
region  represented  by  the  model.  By  comparison  of  the  model  and 
map  it  will  be  seen  that  where  the  slopes  of  the  former  are  steep, 


FIG.  83.  —  Stratification  in  a  sand  dune  near  the  head  of  Lake   Michigan. 

(Bastin.) 

the  lines  of  the  latter  are  close  together,  and  vice  versa.  The  vertical 
distance  between  two  adjacent  contour  lines  is  the  contour  interval. 
The  contour  interval  varies  on  different  maps.  In  regions  of  low 
relief  an  interval  of  10  or  20  feet  is  used  frequently  ;  in  mountainous 
areas  an  interval  of  500  or  more  feet  sometimes  has  to  be  used  in 
order  to  avoid  having  the  lines  too  close  together  to  be  read.  In  the 
map  of  Figure  85  the  interval  is  20  feet,  the  exact  value  of  the  100 
and  200  foot  lines  being  indicated.  By  counting  the  lines  it  will  be 
seen  that  the  top  of  the  hill  to  the  left  of  the  river  is  over  260  feet 
above  the  level  of  the  ocean  in  the  foreground.  It  cannot  be  280 
feet  high,  however,  for  no  280  foot  line  is  drawn.  A  comparison  of 
the  model  and  map  will  show  also  how  valley  depressions  are  shown 
by  contours. 

The  features  shown  upon  the  three-color  contour  maps  are  of 
three  general  classes  :  (1)  elevations  and  irregularities  of  the  sur- 
face, shown  by  brown  contours ;  (2)  water,  including  streams, 
ponds,  lakes,  etc.,  represented  in  blue  ;  (3)  artificial  features,  such 


94  PHYSICAL  GEOLOGY 

as  roads,  railroads,  towns,  boundary  lines,  and  the  like,  indicated 
in  black. 


FIG.  84.  —  Model  of  ideal  landscape.     (U.S.  Geol.  Surv.) 


FIG.  85.  —  Topographic  map  of  ideal  landscape.     (U.S.  Geol.  Sun.) 

Migration  of  dunes.  —  If  winds  strike  the  base  of  a  dune 
with  less  than  they  are  able  to  carry,  as  happens  frequently, 
they  pick  up  sand  from  the  dune  surface  and  move  it  toward 
•or  to  the  crest,  beyond  which  it  falls.  By  this  transfer  of  sand 


PLATE  I.  FIG.  A.  DUNES  ON  THE  COAST  OF  NEW  JERSEY.  Contour 
interval,  10  feet.  Scale,  about  1  mile  per  inch.  (Long  Beach.  New  Jersey, 
Sheet,  U.S.  Geological  Survey.)  FIG.  B.  DUNES  ALONG  THE  ARKANSAS 
RIVER  IN  SOUTHWESTERN  KANSAS.  Contour  interval,  20  feet.  Scale,  about 
2  miles  per  inch.  (Garden,  Kansas,  Sheet,  U.S.  Geological  Survey.) 


96 


PHYSICAL  GEOLOGY 


from  their  windward  to  their  leeward  sides,  dunes  shift  slowly 
in  position  (Fig.  86).     From  the  nature  of  the  migration  it  is 

apparent  that  only 
an  extremely  small 
fraction  of  the  sand 

FIG.  86.  —  Diagram  showing  successive  positions  •     •      ^^  :ft_   fl+  ___. 
of  a  migrating  sand  dune.  On  at  any 

given  time.     Dunes 

have  often  invaded  and  destroyed  farm  lands  and  forests  (Fig. 
87),  and  have  sometimes  buried  towns.     The   easiest   and 
surest    method    of    stopping     ___  _  .......... 

migrating  dunes  is  to  plant 
vegetation  upon  them  (Fig. 
88).  This  fastens  the  sand 
and  protects  it  from  the  wind. 
Topography  of  dune 
areas.  —  Figures  89,  90,  and 
91  show  typical  dune  topog- 


FIG.  87.  —  Dune  advancing  up- 
on a  forest.  Dune  Park,  Ind. 
(Cowles.) 

raphies.  Depressions  are 
sometimes  as  character- 
istic of  dune  areas  as  are 
the  elevations  (Fig.  B, 
Plate  I;  depressions 
shown  by  hachures  within 
closed  contours),  and  are 
formed  in  a  variety  of 
ways.  They  may  be 
scooped  out  by  the  wind. 
More  sand  may  be  de- 
posited around  than  on  a 
given  place,  which  there- 
fore forms  a  depression. 
Shifting  dunes  may  fill 
parts  of  valleys,  whose 


FIG.  88.  —  Planting  beach  grass  to  stop 
the  drifting  of  sand.  Near  Province- 
town,  Mass.  (U.S.  Dept.  Agr.) 


98 


PHYSICAL  GEOLOGY 


FIG.  90.  —  Dunes  in  Colorado  Desert,  Cal. 
(Fairbanks.) 


unfilled  portions  become  depressions  without  outlets.  Such 
depressions  may  be  occupied  by  marshes  and  temporary  ponds 
and  lakes. 

Taking  the  world  as  a  whole,  the  amount  of  sand  deposited 

by  the  wind,  but  not 
forming  distinct  ele- 
vations, probably  far 
exceeds  that  in  dunes. 
Eolian  sandstone. 
-Wind-laid  sand 
may  be  cemented 
into  sandstone, 
though  this  is  less 
likely  to  occur  than 
in  the  case  of  water- 
laid  sand  (Why?). 
Sand  subjected  to  long-continued  action  by  the  wind  consists 
chiefly  of  quartz,  for  the  softer  minerals  have  usually  been 

reduced  to  dust  and  blown    I--T— , 

away.  The  sand  grains 
have  often  been  worn  to 
small  size.  Water-laid 
sand  is  carried  in  suspen- 
sion more  or  less,  and  so 
subjected  to  less  wear ;  its 
particles  are  therefore 
likely  to  be  larger.  Its 
composition,  too,  is  more  often  mixed.  The  origin  of  a  sand- 
stone may  be  revealed  also  by  its  bedding,  by  the  fossils  it 
contains,  and  in  other  ways.  It  has  been  possible  to  determine 
that  the  sand  of  even  very  ancient  sandstones  was  deposited 
by  the  wind. 

Loess.  —  The  wind  has  been  concerned  in  the  deposition 
in  certain  regions  of  loess.  This  is  a  silt,  often  buff-colored, 
that  is  intermediate  in  coarseness  between  sand  and  clay,  and 
whose  particles  are  remarkably  uniform  in  size.  Extensive 


FIG.  91.  —  Dunes  on  the  coast  of  north- 
ern Denmark.     (Engsig.) 


THE  WORK  OF  THE  ATMOSPHERE 


99 


deposits  of  loess  occur  in  the  Mississippi  Basin,  in  parts  of 
Europe,  and  in  China  (Fig.  92),  where  in  places  it  reaches  a 
thickness  of  hundreds  of  feet.  In  this  country,  at  least,  some 
of  the  lowland  loess  appears  to  have  been  deposited  in  part 


FIG.  92.  —  Loess  deposit  in  Shan-si,  China.  The  cafionlike  cut  followed 
by  the  road  has  been  developed  by  the  wear  of  traffic  and  wind.  Two  old 
levels  of  the  road  are  shown  above  the  one  in  use.  The  view  shows  the 
ability  of  loess  to  stand  in  steep  walls.  (Willis,  Carnegie  Institution.} 


100  PHYSICAL  GEOLOGY 

by  streams,  but  the  more  typical  upland  loess  was  probably 
deposited  by  the  wind.  Loess  soils  are  of  great  fertility 
where  moisture  is  sufficient. 

THE  ATMOSPHERE  AND  ROCK  BREAKING 

Since  changes  in  temperature  are  conditioned  by  the  at- 
mosphere, their  effects  upon  rocks  are  considered  here. 
When  water  freezes,  it  expands  about  one  tenth  of  its  volume. 
In  doing  so  it  exerts  great  force,  as  shown  by  the  bursting  of 
strong  pipes  by  water  freezing  in  them.  When  water  which 
nearly  fills  rock  cavities  freezes,  it  may  pry  and  break  off 
pieces,  and  it  may  form  and  enlarge  cracks.  All  unobtrusive 
processes  by  which  exposed  rock  surfaces  break  up  or  decay 
are  processes  of  weathering.  Weathering  by  the  wedge  work 
of  ice  is  obviously  favored  by  many  thawings  and  freezings. 
For  the  maximum  of  weathering  by  this  process,  it  is  desirable 
that  the  ice,  having  exerted  great  pressure  upon  the  rocks  in 

forming,  should  melt 
promptly,  so  that  the 
water  may  penetrate  far- 
ther into  the  enlarged 
cavities,  and  then  freeze 
again.  Accordingly,  the 
wsdge  work  of  ice  is  in 
general  most  important 

FIG.  93.  —  Exfoliation  of  a  granite  •  i  i  ,    , 

bowlder.     (Hole.)  ln  early  and  late  Winter, 

in  moist  regions  that  are 

situated  in  high  middle  latitudes.  In  very  low  latitudes  tem- 
perature changes  never  involve  the  freezing  point,  except 
at  high  altitudes,  while  in  very  high  latitudes  the  temperature 
may  be  below  the  freezing  point  for  weeks  or  months,  continu- 
ously. 

Changes  in  temperature  help  to  break  rocks  in  another 
way.  Rocks  expand  when  heated  and  contract  when  cooled. 
Since  they  are  poor  conductors  of  heat,  it  is  their  surfaces 
which  are  not  only  first,  but  most  affected  by  changes  in 


THE  WORK  OF  THE  ATMOSPHERE 


101 


temperature.  Under  the  heat  of  day  the  more  rapidly  ex- 
panding surface  accordingly  becomes  too  large  to  fit  the  interior, 
and  with  the  fall  of  temperature  in  late  afternoon  and  night 


FIG.  94.  —  Exfoliate 'weathering  in  granite.     Alabama  Hills,  Owens  Valley, 
Cal.     (Trowbridge.) 

the  surface  layer  becomes  too  small  for  the  less  rapidly  shrink- 
ing rock  beneath.  As  a  consequence,  the  surfaces  of  rocks  often 
shell  off  (Figs.  93  and  94),  the  process  being  known  as  exfolia- 


FIG.  95.  —  Summit  of  Pikes  Peak,  Colorado,  showing  broken   character  of 

the  rock.     (R.  T.  Chamberlin.) 
B.  &  B.  GEOL. 7 


102 


PHYSICAL  GEOLOGY 


tion.  Rocks  containing  a  number  of  minerals  are  more 
likely  than  others  to  be  broken  by  changes  in  temperature, 
for  the  different  min- 
erals expand  and  con- 
tract at  different  rates, 
thus  establishing 
strains  within  the  rock. 
Great  and  frequent 
changes  in  the  temper- 
ature of  the  rocks  fa- 


FIG.  96.  —  Serrate  mountain  topography. 
Peaks  of  granite  in  the  Sierra  Nevadas,  near 
Mount  Whitney.  (Trowbridge.) 


vor  their  breaking  by 
this  process,  and  these 
conditions  are  met  best  in  dry  rather  than  moist  regions,  in 
low  rather  than  high  latitudes,  and^at  high  rather  than  low 


FIG.  97.  —  Serrate  mountain  topography.     Peaks  in  the  Mont  Blanc  group 
of  mountains.     (Tairraz.) 

altitudes.  In  deserts  and  on  bare  mountains,  therefore,  the 
process  is  important,  and  in  many  cases  the  latter  are  nearly 
covered  by  loose,  angular  fragments  that  have  been  broken  in 
this  manner  from  the  rocks  beneath  (Fig.  95),  above  which 
sharp,  serrated  peaks  sometimes  rise  (Figs.  96  and  97).  Pieces 
of  rock  loosened  from  steep  slopes  in  this,  or  other  ways,  ac- 
cumulate at  the  bottom  to  form  piles  of  talus  (Fig.  98). 
Some  of  the  mountains  of  the  Great  Basin  region  are  buried 
knee-deep  with  talus. 


THE  WORK  OF  THE  ATMOSPHERE      103 


FIG.  98.  —  Talus  slope  in  the  Snake  River  Canon,   opposite    Enterprise, 

Idaho.     (Russell,  U.S.  Geol.  Surv.) 

What  can  be  inferred  from  the  picture  concerning  the  char- 
acter of  the  topbeds? 

CHEMICAL  WORK 

The  oxygen,  carbon  dioxide,  and  water  vapor  of  the  atmos- 
phere are  very  active  chemically.  Chief  among  the  rock  sub- 
stances with  which  oxygen  unites  is  iron.  This  process 
(oxidation)  is  illustrated  familiarly  by  the  rusting  of  iron  ob- 
jects exposed  in  damp  weather,  the  rust  being  a  chemical 
combination  of  iron,  oxygen,  and  water.  The  brick-red  and 
yellow  colors  of  many  soils  and  rocks  are  due  to  the  oxidized 
condition  of  their  iron.  Among  the  common  minerals  affected 
by  the  process  are  mica,  hornblende,  and  augite,  all  complex 
silicates  containing  iron.  The  union  of  the  carbon  dioxide 
(CO2)  of  the  atmosphere  with  certain  rock  materials  (carbon- 
ation),  is  also  an  important  and  common  process.  For  ex- 
ample, carbon  dioxide  may  unite  with  the  calcium  and  with 
the  iron  of  minerals  containing  these  elements,  to  form  cal- 
cium carbonate  and  iron  carbonate.  The  chemical  union  of 


104  PHYSICAL  GEOLOGY 

the  water  vapor  in  the  air,  or  of  water  after  it  has  fallen  as 
rain,  with  rock  material,  constitutes  hydration. 

Oxidation,  carbonation,  and  hydration  are  all  factors  in 
the  decay  of  rocks.  All  three  involve  an  increase  in  the 
volume  of  the  rock  affected,  and,  unless  something  is  with- 
drawn simultaneously  in  solution,  the  resulting  pressure  tends 
to  make  it  crumble.  The  products  of  the  changes,  as  in  the 
cases  of  the  iron  and  calcium  carbonates  mentioned  above, 
may  be  soluble,  and  are  likely  to  be  carried  away  by  waters 
percolating  through  the  rock.  Their  withdrawal  tends  to 
increase  the  porosity  of  the  rock,  thereby  weakening  it.  One 
of  the  most  important  chemical  reactions  attending  the  decay 
of  igneous  rocks  is  that  by  which  orthoclase,  acted  upon  by 
water  and  carbon  dioxide,  yields  kaolin  (p.  22).  It  may  be 
expressed  as  follows  : 

Orthoclase  +  Water  +  Carbon  dioxide    =  Kaolin  +  Potassium  Carbonate  +  Quartz. 

2  KAlSi3O8  +  2  H2O  +  CO2  =  H^Si.^     +  K2CO3  +  4  SiO2. 


SUMMARY 

The  most  important  phases  of  the  geological  work  of  the 
atmosphere  are  the  following:  (1)  Its  work  as  an  agent  of 
weathering.  Through  its  effect  upon  changes  in  temperature 
it  influences  (a)  the  wedge  work  of  ice,  and  (6)  the  splitting 
of  rocks  by  their  expansion  and  contraction.  The  oxygen, 
carbon  dioxide,  and  water  vapor  of  the  air  unite  chemically 
with  various  rock  substances,  and,  by  so  doing,  contribute  to 
their  decay.  These  processes  prepare  materials  for  removal 
by  various  transporting  agencies.  (2)  The  transportation  and 
deposition  of  fine  material  by  the  wind.  Although  most  ex- 
tensive in  arid  regions,  this  work  has  affected,  first  and  last,  all 
land  surfaces.  Its  aggregate  effect  is  to  lower  the  lands  and 
build  up  the  ocean  bottoms.  (3)  The  abrasion  of  rocks. 
This  is  most  important  in  deserts,  where  the  atmosphere  is 
often  the  chief  agent  of  degradation.  (4)  By  controlling  the 
conditions  of  evaporation  and  precipitation,  the  atmosphere 


THE  WORK  OF  THE  ATMOSPHERE 


105 


makes  possible  the  work  of  streams  and  of  glaciers,  and  the 
existence  of  land  life. 

QUESTIONS 

1.  On  which  side  of  Lake  Michigan  should  dunes  be  best  de- 
veloped ?     Why  ? 

2.  Why  are  the  most  extensive  dune  areas  of  the  Sahara  in  its 
western  portion  ? 

3.  On  which  side  of  an  east  and  west  mountain  range  in  the 
northern   hemisphere    should    rock    splitting    due    to    temperature 
changes  be  most  important?     Why? 

4.  Compare  and  contrast  the  importance  of  rock  splitting  by 
temperature  changes  at    Chicago  and    Denver,  making  clear  the 
reasons  for  the  differences. 

5.  Describe  the  sequence  of  events  recorded  by  Figure  99. 


FIG.  99.  —  View  on  South  Manitou  Island,  Lake  Michigan.     (Russell, 
U.S..  Geol.  Surv.) 

6.  Why  would  it  be  of  value  to  coat  with  tar  the  bottoms  of 
telegraph  poles  in  arid  regions  ?     Why  coat  only  the  bottoms  ? 

7.  How  would  a  considerable  increase  in  moisture  change  the 
geological  processes  in  operation  in  the  Sahara? 

8.  How  might  depressions  made  by  the  wind  be  distinguished 
from  depressions  made  by  rivers  ? 

9.  Why  are  some  wind-swept  rock  surfaces  smooth,  while  others 
are  minutely  pitted  ? 


106 


PHYSICAL  GEOLOGY 


10.    Indicate  three  ways  in  which  the  wind  might  form  mounds 
such  as  those  shown  in  Figure  100. 


FIG.  100.  —  Mounds  near  Iron  Mountain,  Oregon,  due  to  the  work  of  the 
wind.     (Russell,  U.S.  Geol.  Surv.) 

REFERENCES 

BONNET  :    The  Work  of  the  Atmosphere,  in  The  Story  of  Our  Planet, 

pp.  91-102.     (London,  1893.) 
COBB  :  Where  the  Wind  does  the  Work,  in  Nat.  Geog.  Mag.,  Vol.  XVII, 

pp.  310-317. 
CORNISH  :  On  the  Formation  of  Sand  Dunes,  in  Geog.  Jour.,  Vol.  IX, 

pp.  278-309. 
COWLES  :     The  Ecological  Relations  of  the   Vegetation  of  the  Sand 

Dunes  of  Lake  Michigan,  in  Botanical  Gazette,  Vol.  XXVII, 

pp.  95-117,  167-202,  281-308,  361-391. 
DAVIS  :     The  Geographical   Cycle  in  an  Arid   Climate,  in  Jour,   of 

Geol.,  Vol.  XIII,  pp.  381-407. 

GEIKIE,  J.  :   Earth  Sculpture,  pp.  250-265.     (New  York,  1898.) 
HITCHCOCK  :     Controlling    Sand    Dunes    in    the    United    States    and 

Europe,  in  Nat.  Geog.  Mag.,  Vol.  XV,  pp.  43-47. 
MERRILL  :    The  Principles  of  Rock   Weathering,  in  Jour,  of  Geol., 

Vol.  IV,  pp.  704-724,  850-871. 

-  Rocks,  Rock  Weathering,  and  Soils.     (New  York,  1897.) 
UDDEN  :    Erosion,   Transportation,  and  Sedimentation  performed  by 

the  Atmosphere,  in  Jour,  of  Geol.,  Vol.  II,  pp.  318-331. 


CHAPTER  IV 
THE   WORK   OF   WATERS   UNDERGROUND 

FACTS  ABOUT  GROUND  WATER 

WHEN  a  well,  mine,  or  other  opening  of  sufficient  depth  is 
made  in  the  ground,  water  seeps  into  it  from  the  surround" 
ing  rocks.  The  existence  of  water  beneath  the  surface  (ground 
water)  is  also  proved  in  a  simple  way  by  the  fact  that  it  issues 
from  the  ground  to  form  great  numbers  of  springs  in  all 
humid  regions,  and  occasional  ones  even  in  very  dry  regions. 
By  digging  deep  enough  it  is  possible  almost  anywhere  to 
reach  a  level  where  the  rocks  are  saturated  with  water.  The 
level  below  which  the  rocks  are  full  of  water  is  the  level  of 
ground  water,  or  the  water  table. 

Source  of  ground  water.  —  The  ground  water  is  related 
intimately  to  the  rainfall,  for  water  stands  higher  in  wells 
in  rainy  seasons  than  in  periods  of  drought,  and  springs  are 
more  numerous  and  of  greater  volume  after  plentiful  rains 
than  during  periods  of  dry  weather.  Indeed,  no  other 
source  exists  from  which  important  contributions  to  the 
ground  water  can  be  made.  Most  of  the  water  beneath 
the  ground  therefore  probably  once  fell  as  rain. 

Proportion  of  the  rainfall  which  enters  the  ground.  —  A 
portion  of  the  water  which  falls  as  rain  runs  directly  off  the 
surface  (the  immediate  run-off),  another  part  is  evaporated, 
and  a  third  sinks  into  the  ground.  The  proportion  of  the 
rainfall  which  enters  the  ground  varies  at  different  points 
and  at  different  times,  with  (1)  the  slope  of  the  ground, 
(2)  the  porosity  of  the  soil  and  rocks,  (3)  the  amount  of 
water  already  in  the  rocks;  (4)  the  rate  of  precipitation, 

107 


108  PHYSICAL  GEOLOGY 

(5)  the  amount  and  character  of  vegetation  upon  the  surface, 
and  for  other  less  important  reasons.  The  greater  the  slope 
of  the  surface,  the  larger  the  proportion  of  the  rain  which 
joins  the  run-off,  and  the  smaller  the  proportion,  conse- 
quently, which  enters  the  ground.  If  the  spaces  between 
the  rock  particles  are  large,  as  in  sand  or  gravel,  more  water 
sinks  into  the  ground  than  when  the  surface  material  is  dense 
and  compact,  like  clay.  If  the  rocks  are  already  nearly  or 
quite  full  of  water,  little  or  no  more  can  enter.  After  the 
surface  material  is  filled  with  water,  no  more  can  enter  until 
that  within  sinks  out  of  its  way.  Meanwhile,  all  the  water 
that  falls  is  disposed  of  in  some  other  manner.  Other  things 
equal,  therefore,  most  water  sinks  into  the  ground  when  the 
downfall  is  gentle.  This  fact  is  in  part  responsible  for  the 
familiar  statement  that  gentle  rains  are  more  beneficial  to 
crops  than  heavy  downpours.  The  run-off  is  greater  from 
bare  slopes  than  from  slopes  clothed  with  vegetation ;  in  the 
latter  case,  accordingly,  a  greater  proportion  of  the  rain 
sinks  below  the  surface.  This  fact  is  a  fundamental  considera- 
tion in  connection  with  the  recent  agitation  in  favor  of  forest 
reserves  about  the  sources  of  rivers  which  afford  navigation 
or  water  power.  With  such  forests  a  larger  proportion  of 
the  rainfall  sinks  beneath  the  surface,  later  to  issue  gradually 
as  seepage  and  springs,  and  so  maintain  the  volume  of  the 
rivers  throughout  the  year.  Without  them,  the  water  from 
spring  rains  and  melting  snows  flows  away  quickly,  often 
causing  destructive  floods,  and  leaves  the  rivers  with  greatly 
diminished  volume  in  the  dry  season. 

The  ground  water  at  any  given  place  is  not  dependent 
entirely  upon  local  rainfall,  for,  as  noted  below,  water  often 
flows  great  distances  underground. 

Depth  to  which  ground  water  descends.  —  From  what 
has  preceded,  it  is  evident  that  water  must  fill  the  cavities 
in  the  rocks  from  the  water  table  down  as  far  as  openings 
exist.  As  already  indicated  (p.  77),  this  is  believed  to  be 
to  a  distance  of  five  or  six  miles  only,  for  at  some  such  depth 


THE  WORK  OF  WATERS  UNDERGROUND        109 

all  spaces,  however  small,  are  closed  by  the  tremendous 
weight  of  the  overlying  rocks. 

The  temperature  of  the  rocks,  and  therefore  of  the  ground 
water,  increases  with  the  depth.  The  rate  of  increase  varies 
in  different  places  from  one  degree  for  about  17  feet  of 
descent  to  one  degree  for  over  100  feet.  The  larger  figure 
is  probably  much  nearer  the  average  than  the  smaller  one. 

Amount  of  ground  water.  —  If  the  average  distance  from 
the  water  table  to  the  base  of  the  zone  of  fracture  were 
known  definitely,  and  if,  in  addition,  it  were  possible  to 
determine  what  proportion  of  the  volume  of  the  rocks  be- 
tween is  made  up  of  cracks,  pores,  etc.,  it  would  be  possible 
also  to  determine  accurately  the  total  amount  of  the  ground 
water.  It  is  sufficient  to  form  a  layer  of  water  over  the 
surface  of  the  earth,  having  a  depth  estimated  variously  at 
from  800  feet,  and  less,1  to  3500  feet,  and  more.  The  larger 
figure  would  represent  only  about  one  third,  and  the 
smaller  figure  but  a  small  fraction  of  the  water  of  the  oceans. 
The  ground  waters  encircle  the  earth,  forming  a  rude  sphere. 
The  name  applied  to  the  waters  of  the  earth,  the  hydro- 
sphere, is  accordingly  an  appropriate  one. 

Form  and  position  of  the  water  table.  —  The  water  table 
is  not  a  level  surface.  If  the  rocks  beneath  an  uneven  sur- 
face such  as  that  shown  in  Figure  101  were  filled  with  water 
by  rains,  the  water  * 
table  would,  of  course, 
coincide  with  the  sur- 
face of  the  ground,  and 
would  be  far  from 

level         Subsequently     FlG<  101<  —  Diagram  showing  the  relation  of 

*'*        the  level  of  ground  water  (the  broken  line) 

the  Water  WOUld  move       to  the  surface  of  the  ground  and  to  a  lake  and 

under     gravity     from      river- 

the  higher  levels  A  and  B  toward  the  neighboring  lower  levels. 
The  ultimate  tendency  would  be  to  make  the  ground-water 
surface  level.  The  movement  of  the  water  would,  however, 

1  One  of  the  latest  estimates  fixes  the  depth  at  only  96  feet. 


HO  PHYSICAL  GEOLOGY 

be  extremely  slow,  because  of  the  small  passages  through 
which  it  must  move  and  the  friction  it  would  develop  with 
the  rocks.  Long  before  the  surface  of  the  water  became 
level,  further  rains  would  be  likely  to  raise  it  again  beneath 
the  uplands.  In  keeping  with  these  considerations,  the 
water  table  is  found  to  repeat,  in  a  general  way,  the  topog- 
raphy of  the  surface  above.  As  Figure  101  suggests,  how- 
ever, it  is  nearer  the  surface  below  the  valley  bottoms  than 
it  is  beneath  the  hilltops. 

As  implied  in  the  preceding  paragraph,  the  position  of  the 
water  table  varies  not  only  from  place  to  place,  but  also 
from  time  to  time  at  any  given  place.  It  is  higher  after 
heavy  rains,  and  lower  during  periods  of  drought. 

How  ground  water  is  disposed  of.  —  On  the  average  the 
amount  of  water  withdrawn  from  the  ground  in  the  course 
of  a  year  probably  balances  that  which  enters.  It  is  with- 
drawn in  various  ways.  It  issues  as  springs  and  as  seepage, 
is  pumped  out  through  wells,  flows  underground  to  the  sea, 
is  taken  up  by  plants,  and  evaporates  into  the  air  which 
fills  the  rock  cavities  above  the  water  table.  It  sometimes 
enters  into  chemical  combination  with  rocks  (pp.  103-104). 
The  deeper  water  is  probably  imprisoned  underground  for  long 
periods. 

SPRINGS  AND  UNDERGROUND  CIRCULATION 

Kinds  of  springs.  —  When  water  issues  from  the  ground 
in  volume  sufficient  to  form  a  distinct  current,  it  constitutes 
a  spring.  When  it  issues  in  less  quantity,  it  is  known  as 
seepage.  Springs  differ  in  many  respects,  and  these  differ- 
ences have  led  to  numerous  classifications.  Thus  there  are 
hot  (thermal)  and  cold  springs,  intermittent  and  constant 
springs,  deep  and  shallow  springs,  and  many  others. 
Medicinal  springs  are  those  whose  waters  have  real  or  sup- 
posed medicinal  value.  The  springs  at  Saratoga  Springs, 
New  York;  Hot  Springs,  Arkansas;  Vichy,  in  central  France; 
and  Karlsbad,  in  Bohemia,  are  famous  examples.  These 


THE  WORK  OF  WATERS  UNDERGROUND        111 

cities  grew  up  largely  or  wholly  because  of  the  value  of  their 
medicinal  waters.  In  general,  the  waters  of  mineral  springs 
either  contain  large  quantities  of  mineral  matter  in  solution, 
or  something  which  is  striking  because  of  its  taste  or  odor. 
Most  medicinal  springs  are  mineral  springs,  but  the  reverse 
is  not  true.  In  1908  there  were  695  commercial  mineral 
springs  in  the  United  States,  which  sold  56,108,820  gallons 
of  mineral  waters,  valued  at  $7,287,269. 

Among  the  more  common  deposits  of  springs  are  lime 
carbonate  by  calcareous  springs,  and  iron  compounds  by 
ferruginous  springs.  Silica,  gypsum,  and  many  other  things 
are  also  deposited  by  springs.  Limestone  is  deposited  by 
springs  at  San  Filippo,  in  Tuscany,  at  the  rate  of  one  foot 
in  four  months,  and  has  formed  a  mass  250  feet  thick,  a 
mile  and  a  quarter  long,  and  a  third  of  a  mile  wide.  Each 
year  the  springs  at  Bath,  England,  discharge,  on  the  average, 
mineral  matter  sufficient  to  cover  an  area  of  11,340  square 
feet  with  a  layer  one  foot  deep.  A  spring  near  Minden, 
Germany,  has  been  found  to  bring  to  the  surface  each  year 
salt  enough  to  form  a  solid  cube  measuring  72  feet  on  a  side. 

Hillside  springs,  flowing  wells,  deep-seated  springs,  and 
geysers  are  types  of  springs  which  may  be  discussed  briefly. 

Hillside  springs.  —  Figure  102  illustrates  the  occurrence 
of  the  ordinary  hillside  spring.  If  the  upper  (P)  material  is 
relatively  porous 
(for  example, 
sand),  while  the 
lower  (/)  material 
is  relatively  im- 
pervious (like 

Clay),    rain    Water     FIG.  102.  -  Diagram  showing  conditions  for  ordi- 
J  ' ;  nary  hillside  springs. 

will     penetrate 

readily  to  the  sloping  surface  of  the  impervious  beds,  along 
which  it  will  flow  slowly,  and  issue  as  springs  at  the  base  of 
the  hill.  The  great  majority  of  springs  are  of  this  class. 
They  are  usually  not  of  great  volume. 


112 


PHYSICAL  GEOLOGY 


FIG.    103.  —  Diagram    illustrating    the    conditions 
necessary  for  an  artesian  well. 


Flowing   wells.  —  These   are   artificial   springs,    and  were 
formerly  called  artesian  wells,   from  a  province   in  France 

(Artois),  where 
the    first     ones 
^j^j^-s^K^si  ^  ^-^-^  were    dug.      Any 

\ 


very  deep  well  is 
now  called  an  ar- 
tesian well,  even 
though  its  water 
has  to  be  pumped 
to  the  surface. 
The  conditions 
necessary  for  the 
formation  of  flow- 
ing wells  are  shown  in  Figure  103.  They  are:  (1)  a  porous, 

water-carrying  layer  (A),  which  outcrops  at  a  level  (B)  higher 

than  that  of  the  mouths  of  the  wells ;  (2)  rainfall  sufficient  to 

keep  the  porous  layer  well  filled 

with  water;    (3)    an   impervious 

layer  (C)  above  the  porous  layer. 

This  confines  to  the  latter  the  ma- 
jor part  of  the  water  which  enters 

it  at  B.    If  under  these  conditions 

wells  are  sunk  to  the  porous  layer 

as  indicated,   the  water  at  the 

points  tapped,  which  is  under  the 

pressure  of  a  sloping  stratum  of 

water  which  fills  the  cavities  of 

the  porous  layer  and  reaches  to 

the  water  table  near  B,  will  be 

forced  through  the   openings  to 

the  surface,  forming  flowing  wells. 

If  a  boring  is  made  too  far  from 

the  water  table  near  the  outcrop 

of  the  porous  layer,  the  loss  of 


FIG.  104.  —  An  artesian  well 
at  Lynch,  Neb.  Flows  more 
than  3000  gallons  per  min- 
ute. (Barton,  U.S.  Geol. 
Sun.) 


force  by  friction  between  the  water  and  the  rocks  through 


THE  WORK  OF  WATERS  UNDERGROUND        113 

which  it  passes  will  come  to  equalize  the  pressure  of  the  water 
column,  and  no  outflow  will  occur.  Figure  104  shows  a 
typical  flowing  well  of  large  volume. 

Artesian  wells  are  common  in  parts  of  the  Atlantic  Coastal 
Plain,  the  Great  Plains,  southern  Wisconsin,  northern 
Illinois,  and  in  many  other  places.  Many  cities  such  as 
Savannah,  Georgia,  and  Brooklyn,  New  York,  whose  wells 
have  a  capacity  of  about  22,000,000  gallons  per  day,  receive 
much  water  supply  from  artesian  wells.  In  parts  of  the  West, 
artesian  waters  are  used  extensively  for  irrigation  purposes. 

Deep-seated  springs.  —  Ground  water  ascends  to  the 
surface  from  relatively  great  depths  through  large  cracks 
or  fissures  in  the  rocks. 
It  rises  by  hydrostatic 
pressure,  as  in  the  case 
of  flowing  wells.  Fig- 
ure 105  illustrates  the 
occurrence  of  a  deep- 
seated  spring  and  sug- 
gests the  intricate 
courses  through  joints 

and    Other    cracks    that    FIG.  105.  —  Diagram  showing  the  intricate 
water    which     descends        underground   drainage  which  issues  in   a 
.  ,        ,  ,        ,      , .  deep-seated  spring.     (Geikie.) 

to  considerable   depths 

must  follow  before  completing  its  underground  circulation. 
It  moves  down  rather  readily  through  the  surface  rocks,  but 
as  it  descends  deeper  the  pressure  of  the  column  of  water 
behind  it  may  force  it  toward  the  surface  through  any  open- 
ings it  encounters  which  lead  upward.  Sinking  again  through 
other  cracks,  it  may  follow  a  zigzag  lateral  course  for  a  long 
distance,  moving  repeatedly  up  and  down  before  reaching  at 
last  a  fissure  or  other  trunk  channel  through  which  it  may 
rise  to  the  surface. 

Geysers.  —  Geysers  are  hot  springs  that  erupt  at  intervals. 
Existing  geysers  are  confined  to  areas  of  recent  volcanic 
activity  in  Yellowstone  Park,  Iceland,  and  New  Zealand. 


114 


PHYSICAL  GEOLOGY 


At  irregular  intervals  the  expansion  of  steam  formed  at 
some  point  below  the  surface  of  the  water  of  the  geyser, 
supposedly  by  contact  with  hot  volcanic  rocks,  forces  the 
overlying  column  of  water  into  the  air,  like  a  great  fountain. 
As  the  lava  which  heats  the  water  cools,  eruptions  become 
less  and  less  frequent,  and  finally  cease.  All  existing  geysers 
will  therefore  become  extinct.  They  may,  however,  be  re- 
placed by  others  elsewhere. 

THE  WORK  OF  GROUND  WATER 

Mechanical  work.  —  In  general,  ground  water  transports 
very  little  material  in  a  solid  state,  and  this  material  wears 


FIG.  106.  —  Shows  the  results  of  creep.  The  bare  rock  is  sandstone,  resting 
on  a  sloping  surface  of  shale  which  is  often  made  wet  and  more  or  less  slip- 
pery by  water  sinking  through  the  joints  and  other  openings  of  the  rocks 
above.  The  detached  mass  of  sandstone  has  slowly  settled  away  from 
the  rock  wall  on  the  left,  the  line  of  division  being  determined  by  a  large 
joint  plane. 

the  rock  surfaces  it  encounters  but  slightly.  This  is  because 
ground  water  is  rarely  concentrated  in  well-defined  channels 
to  form  streams,  but  usually  moves  very  slowly  (a  few  feet 


THE  WORK  OF  WATERS  UNDERGROUND 


a  day)  and  at  any  given  point,  in  small  volume.     By  making 

the  rocks  which  it  saturates  heavier  and  sometimes  slippery, 

ground  water  often  assists 

gravity  in  moving   mate- 

rial, sometimes  masses  of 

great    size,    down    slopes. 

When    the    movement   is 

too  slow  to  be  seen,  it  is 

called  creep  (Figs.  106  and 

107),  when  sudden,  a  land- 

slide or  slump  (Figs.  108 

and   109).      When   slopes 

are  of   unprotected    clay, 

ground    water    influences 

creep  as  follows.     As  the 

surface    clay    dries    after 

rains,  it  shrinks  and  cracks, 

forming  sun  cracks.     The 

opening   of    a    horizontal 

crack  is  largely  the  result 

of  the  down-slope  move- 

ment of  the   clay  below, 

rather  than  the  up-slope 

movement  of  that  above, 

for  gravity   assists   downward  movement,  while  it  opposes 

movement  up-slope.     With  the  first  shower  the  clay  swells 

and  the  crack  is   closed.     Under  the   influence   of  gravity 

it  is   closed   chiefly   from   above   rather  than  from   below. 

Repeated   shrinking   and   swelling    mean   very  slow  move- 

ment   down    slope.     Other    factors    besides    ground   water 

are    involved   with    gravity   in   creep.     For  example,   rock 

fragments  on  a  slope  expand  under  the  heat  of  day,  and,  be- 

cause of  the  influence  of  gravity,  the  expansion  is  chiefly 

downward.     When  the   fragments  cool,  they  contract,  and 

largely  from  their  up-slope  ends,  since  this  again  involves 

movement  with,  rather  than  against,  gravity.     The  result  of 


FIG.  107.  —  The  side  of  a  ravine  near 
Crawfordsville,  Ind.,  showing  trees 
leaning  down-slope,  in  part  because 
of  creep.  The  surface  material  creeps 
faster  than  that  at  a  slight  depth,  tip- 
ping the  trees  toward  the  axis  of  the 
ravine.  (Forestry  and  Irrigation.) 


116  PHYSICAL  GEOLOGY 

many  expansions  and  contractions  is  an  appreciable  move- 
ment toward  the  base  of  the  slope.  Landslides,  too,  may  be 
due  to  a  combination  of  causes.  Rock  masses  may  be  in  an 
unstable  condition,  and  traversed  by  fissures  and  joint 
planes  in  such  a  manner  as  to  favor  landslides.  Among 
other  things,  earthquake  tremors  and  sudden  and  consider- 
able changes  in  temperature  (especially  if  they  involve  the 


FIG.  108.  —  Landslide  in  the  San  Juan  Mountains,  Colo.     (Howe,  U.S. 
Geol.  Sun.) 

freezing  point)  may  determine  the  moment  when  such  masses 
slump. 

Although  the  mechanical  work  of  ground  water  is  rela- 
tively unimportant,  it  does  a  vast  amount  of  chemical  work 
of  several  sorts,  to  which  attention  is  now  directed. 

Solution  by  ground  water.  —  Pure  water  dissolves  only  a 
few  minerals  and  rocks  readily,  but  practically  all  ground 
waters  are  impure.  In  general,  water  dissolves  mineral 
matter  more  easily  when  its  temperature  is  high  rather  than 
low,  and  when  it  is  under  great,  rather  than  little,  pressure. 
Its  power  to  dissolve  certain  rock  material  is  also  increased 


THE  WORK  OF  WATERS  UNDERGROUND        117 


greatly  when  it  contains  carbon  dioxide  dissolved  from  the 
air,  and  to  some  extent  when  it  contains  matter  derived 
from      decaying      vegetation.          ^ 
Since  temperature  and  pressure          sE 
increase  with  depth,  it  might          ^d 
at  first   thought   appear  that 
solution     by    ground     waters 
should  be  most  important  near 
the  base  of  the  zone  of  frac- 
ture.    It  is  evident,  however, 
that   for  the   continuation  of 
this  work  there  must  be  active 
circulation  of  the  water :  water  FlG-  109-  —  Profile  of  Turtle  Moun- 

,         .   .  .  tain,  Alberta,  showing  the  amount 

Charged    With    mineral   matter  Of  material  removed  in  the  Frank 

in  Solution  must  be  withdrawn  landslide  of  Apr.  29,  1903.     A  mass 

.,  of  rock  nearly  half    a  mile  square 

to  make  room  for  other  water  and  some  400  to  500  feet  thick  in 
able  to  dissolve  more.  As  in- 
dicated on  page  80,  the  zone  of 
greatest  solution  is  accordingly 
near  the  surface,  where  circula- 
tion is  most  active.  Below  this  the  dominant  thing  is  deposi- 
tion, rather  than  solution.  It  is,  of  course,  not  to  be  inferred 
that  no  solution  occurs  in  the  lower  zone  of  deposition,  or 
that  deposition  does  not  occur  in  the  zone  of  solution. 

Rocks  are  made  porous  and  weakened  by  the  withdrawal 
in  solution  of  their  soluble  constituents,  and  solution  is  one 
of  the  most  important  processes  in  the  weathering  of  rocks. 
Thus,  if  its  cement  be  removed  in  solution,  sandstone  crumbles 
into  sand,  and  conglomerate  into  gravel.  Carbonated  ground 
water  (water  containing  carbon  dioxide)  sometimes  removes 
so  much  limestone  in  solution  from  a  given  place  that  a 
cave  or  cavern  is  formed  (Fig.  110).  Such  caves  are  com- 
mon in  parts  of  Kentucky  and  Indiana.  There  are  said 
to  be  over  100,000  miles  of  underground  chambers  in 
the  limestone  rocks  of  the  former  state.  The  thinned  and 
weakened  roofs  of  caverns  may  fall  in,  forming  surface 
B.  &  B.  GEOL.  —  8 


places  broke  suddenly  from  the 
east  face  of  the  mountain  and  de- 
scended with  great  violence  to  the 
valley  below,  destroying  part  of  the 
town  of  Frank. 


118  PHYSICAL  GEOLOGY 

depressions  called  sinks.  Sinks  are  often  formed,  however, 
without  the  collapse  of  cavern  roofs.  Many  are  funnel- 
shaped  depressions  dissolved  in  the  rocks,  through  which 
water  runs  down  into  a  joint,  which  may  lead  to  a  cavern 
(Fig.  110).  If  the  two  ends  of  a  roof  collapse,  while  the 
middle  portion  remains,  the  latter  constitutes  a  natural 
bridge.  Natural  bridges  are  also  formed  in  other  ways 
(p.  244).  In  Karst,  on  the  eastern  side  of  the  Adriatic  Sea, 
the  limestone  rocks  are  so  honeycombed  by  tunnels  and 
openings  dissolved  out  by  ground  waters,  that  much  of  the 


FIG.  110.  —  Diagram  of  caverns  and  sinks. 

drainage  is  underground.  Large  sinks  abound,  some  of  them 
five  or  six  hundred  feet  deep.  Streamless  valleys  are  com- 
mon, and  valleys  containing  streams  often  end  abruptly 
where  the  latter  plunge  into  underground  tunnels  and  caverns, 
sometimes  to  reappear  as  great  springs  elsewhere.  Irregular 
topography  of  this  kind,  developed  by  the  solution  of  surface 
and  ground  waters,  is  known  as  karst  topography,  after  the 
type  region  in  Austria-Hungary. 

Much  of  the  material  dissolved  by  the  actively  circulating 
ground  water  of  the  upper  zone  is  sooner  or  later  carried  to 
the  sea.  It  has  been  estimated  that  the  rivers  carry 
4,975,000,000  tons  of  mineral  matter  to  the  sea  in  solution 
yearly,  and  most  of  this  is  contributed  by  ground  water. 
It  has  also  been  calculated  that  the  salt  in  solution  in  the 
sea  would,  if  it  were  taken  out  of  solution  and  deposited, 
form  a  layer  175  feet  in  thickness  over  the  bottom,  —  that 
is,  over  nearly  three  fourths  of  the  earth's  surface.  Most  of 


THE  WORK  OF  WATERS  UNDERGROUND        119 

this,  and  in  addition  that  contained  in  the  waters  of  salt 
lakes  and  in  the  great  salt  deposits  of  certain  rock  forma- 
tions, was  obtained  by  ground  water  from  rocks  that  con- 
tained the  constituents  of  salt.  Again,  the  great  formations 
of  limestone  were  formed  either  by  the  accumulation  of  the 
shells  of  marine  animals  that  take  lime  carbonate  from  solu- 
tion in  the  ocean  water,  or  by  precipitation  from  the  over- 
charged waters  of  embayments  or  lagoons  of  the  sea  (p.  39). 
Most  of  the  material  of  the  limestone  was  dissolved  by 
ground  water  from  the  rocks  of  the  land  and  delivered  to 
the  streams,  by  which  it  was  carried  to  the  sea.  These  facts 
help  to  illustrate  the  importance  of  the  work  of  solution 
done  by  ground  water.  The  effect  of  the  transfer  of  dis- 
solved material  to  the  sea  is  to  lower  the  land.  This  is  being 
accomplished  by  solution  alone  at  an  estimated  average  rate 
of  one  foot  in  about  13,000  years. 

Deposition  by  ground  water.  —  While  possibly  the  major 
part  of  the  mineral  matter  dissolved  by  ground  water  is 
carried  in  solution  directly  to  the  sea,  vast  quantities  are 
deposited  below  and  at  the  surface  (p.  80).  Deposition 
may  be  brought  about  in  several  waj^s.  The  water  may  be 
overcharged  and  deposit  because  of  (1)  evaporation,  (2)  a 
lowering  of  temperature,  (3)  a  decrease  in  pressure,  (4)  a 
loss  of  part  or  all  of  the  gas  it  contained,  or  (5)  the  mixing 
of  waters  having  different  things  in  solution.  In  the  last 
case  new  combinations  are  likely  to  be  formed,  leaving  the 
water  overcharged  with  one  or  more  things,  which  are  de- 
posited. Certain  minute  plants  also  have  the  power  of  ex- 
tracting some  things  from  solution.  As  already  indicated, 
the  zone  of  greatest  cementation  lies  below  the  zone  of  solu- 
tion, where  the  ground  water  is  sluggish,  and  heavily  charged 
with  mineral  matter. 

Where  material  is  deposited  among  loose  rock  particles, 
the  latter  may  be  cemented  into  firm  rock  (p.  37).  Where 
deposition  from  solution  occurs  on  the  walls  of  cracks  or 
fissures,  the  material  forms  mineral  veins  (Fig.  111).  Most 


120 


PHYSICAL  GEOLOGY 


of  the  copper,  lead,  zinc,  iron,  gold,  and  silver,  occurring  as 
veins  or  otherwise,  was  scattered  widely  through  the  neigh- 


FIG.  111.  —  Calcite  veins  in  volcanic  tuff.     West  of  Kincraig  Point,  Elie, 
Fife.     (H.M.  GeoL  Sun.) 

boring  rocks,  usually  in  the  form  of  some  compound,  and 
was  dissolved,  concentrated,  and  deposited  in  its  present 
position  by  percolating  waters.  Deposits  made  in  caves  are 

interesting,  but  of  little 
importance.  Those  which 
extend  downward  from 
the  roof,  icicle  fashion, 
are  stalactites;  those 
which  are  built  upward 
from  the  floor  are  stalag- 
mites (Fig.  112).  In  cer- 
tain arid  parts  of  the 
West,  ground  water  as- 
cends to  the  surface  and 
is  evaporated  there,  leav- 
ing the  material  which  it 
held  in  solution  as  an 
incrustation  which  in 
Places  covers  large  areas. 
Ground  water  some- 


THE  WORK  OF  WATERS  UNDERGROUND       121 

times  dissolves  material  scattered  through  rocks  and  brings  it 
together  and  deposits  it  in  nodular  masses.  Such  concretions 
(Fig.  113)  are  usually  of  material  different  from  the  dominant 
material  of  the  rocks  in  which  they  occur.  Thus,  concretions 
in  limestone  are  frequently  of  silica,  and  concretions  in  clay 
or  shale  are  often  of  calcium  carbonate  or  an  iron  compound. 
Concretions  vary  in  form  from  nearly  perfect  spheres  to  nota- 


FIG.  113.  — Ironstone  concretion  in  shale.     South  shore  of  Lake  Huron,  about 

25  miles  northeast  of  Sarnia,  Ont.     (Lee.) 

How  does  the  picture  prove  that  the  concretion  was  formed  after 
the  deposition  of  the  shale  ? 

bly  irregular  lumps  and  masses,  and  range  in  diameter  from 
a  fraction  of  an  inch  to  10  or  15  or  more  feet.  A  fossil 
sometimes  forms  the  nucleus  about  which  a  concretion  grows. 
While  many  concretions  have  been  made  after  the  formation 
of  the  inclosing  rocks,  others  have  developed  during  the 
deposition  of  the  sediments.  When  concretions  are  made 
in  soft  sediments,  they  often  press  the  surrounding  material 
away  as  they  grow. 

Small  cavities  in  rocks  may  be  partly  or  wholly  filled  by 
material  deposited  from  solution  in  ground  water,  forming 


122  PHYSICAL  GEOLOGY 

secretions.    Cavities  lined  with  inward-pointing  crystals  are 
geodes.    Agates  are  also  examples  of  secretions. 

Other  changes  accomplished  by  ground  water.  —  The 
shells  of  animals,  trunks  of  trees,  etc.,  which  are  buried  in 
gathering  sediments,  may,  as  they  decay,  be  carried  away 
bit  by  bit  by  ground  waters  and  be  replaced  by  the  deposition 
from  solution  of  other  mineral  matter,  often  silica.  The 


FIG.   1 14.  —  Petrified  logs  in    the    petrified    forest,   near  Adamana,   Ariz. 

(Atwood.) 

minutest  structure  of  the  wood,  for  example,  is  thus  pre- 
served in  stone,  and  the  wood  is  said  to  be  petrified  (Fig.  114). 
The  process  is  called  petrifaction.  It  has  been  of  great  im- 
portance in  preserving  in  fossil  form  a  record  of  past  life. 
Replacement  by  ground  waters  has  affected  many  kinds  of 
rocks  on  a  vast  scale.  Many  concretions  are  made  by  the 
process  of  replacement. 

As  ajready  pointed  out  (p.  104),  ground  water  may  enter 
into  chemical  combination  with  certain  rock  constituents. 
Hydration  is  a  chief  factor  in  the  decay  of  rocks.  Mineral 
matter  in  solution  in  percolating  water  may  also  form  new 
chemical  combinations  with  constituents  of  the  rocks  through 
which  the  water  passes,  and  thus  be  locked  up  for  long 
periods  of  time. 


THE  WORK  OF  WATERS  UNDERGROUND        123 

Importance   of  chemical  work  of  ground  water.  —  From 

the  preceding  paragraphs  it  is  apparent  that  ground  water 
may  modify  the  character  of  rocks  in  several  ways:  (1)  by 
removing  soluble  constituents;  (2)  by  depositing  new  ma- 
terial in  rock  cavities;  (3)  by  replacing  old  material  with 
new;  and  (4)  by  forming  new  chemical  combinations.  The 
result  is  often  to  alter  profoundly  the  character  of  the  rocks 
affected.  Thus  water  is  one  of  the  leading  agents  of  meta- 
morphism  (p.  78).  Furthermore,  changes  of  the  sort  sug- 
gested above  must  have  occurred  on  a  vast  scale,  for  ground 
waters  have  been  at  work  throughout  the  zone  of  fracture 
for  untold  millions  of  years. 

QUESTIONS 

1.  In  order  to  contain  water  permanently,  must  wells  be  sunk 
deeper  below  the  surface  in  valleys  or  on  uplands  ? 

2.  Make  a  diagram  showing  (1)  the  position  of  the  water  table 
during  the  rainy  and  during  the  dry  months,  and  (2)  two  wells,  one 
of  which  goes  dry  at  times,  while  the  other  always  contains  water. 


FIG.  115.  —  An  exposure  of  rocks  in  a  railroad  cut,  Columbia,  S.  C. 
(Trowbridge.) 

3.  Describe  the  characteristics  of  a  climate  that  should  (1)  favor, 
and  (2)  hinder,  the  work  of  solution  by  ground  water. 

4.  Would  caves  be  more  likely  to  develop  in  limestone  regions 
whose  surfaces  were  well  above  or  near  to  sea  level  ? 

5.  Why  cannot  extensive  caverns  in  a  given  region  be  formed 


124  PHYSICAL  GEOLOGY 

below  the  level  of  the  bottom  of  the  largest  valley  of  the  region  ? 
(See  Fig.  110.) 

6.  In  parts  of  eastern  Tennessee  sinks  occur  in  belts  that  are 
rudely  parallel  to  one  another.     What  facts  may  be  inferred  from 
this  concerning  (1)  the  character,  and  (2)  the  structure,  of  the  rocks  ? 

7.  What  inferences  may  be  made  from  the  fact  that  on  a  given 
valley  side  strong  springs  occur  at  intervals  along  a  line  that  rises 
down  valley  ? 

8.  What  facts  concerning  ground  water  are  illustrated  by  Figure 
115?     What  inferences  may  be  made  concerning  the  rocks? 

9.  What  inferences  concerning  ground  water  may  be  made  from 
the  fact  that  at  many  points  in  the  ocean  near  shore,  strong  fresh- 
water springs  well  up  ? 

REFEKENCES 

BLATCHLEY  :    Indiana  Caves  and  Their  Fauna,  in  21st  Ann.  Kept., 

Ind.  Geol.  Surv.,  pp.  121-212. 
CHAMBERLIN  :   Artesian  Wells,  in  Geology  of  Wisconsin,  Vol.  I,  pp. 

689-701. 
—  Requisite  and  Qualifying  Conditions  of  Artesian  Wells,  in  5th 

Ann.  Kept.,  U.S.  Geol.  Surv.,  pp.  131-173. 

CHITTENDEN  :    The  Yellowstone  National  Park.      (Cincinnati,  1895.) 
GEIKIE,   J.  :    Land  Forms   Modified   by  the  Action  of   Underground 

Water,  in  Earth  Sculpture,  pp.  266-277.     (New  York,  1898. ) 
HOVEY  :    Celebrated  American  Caverns.     (Cincinnati,  1896.) 
HOVEY  and  CALL  :    The  Mammoth  Cave  of  Kentucky.     (Louisville, 

1897.) 
MERRILL:    The  Principles  of  Rock  Weathering,  in  Jour,  of  Geol., 

Vol.  IV,  pp.  704-724,  850-871. 

-  Rocks,  Rock  Weathering,  and  Soils.     (New  York,  1897.) 
SHALER  :   Caverns  and  Cavern  Life,  in  Aspects  of  the  Earth,  pp.  98- 

142.     (New  York,  1889.) 


CHAPTER  V 

THE   WORK   OF   STREAMS 

The  run-off.  —  It  is  estimated  that  the  total  amount  of 
rain  which  falls  yearly  on  all  the  land  is  equal  to  about 
35,000  cubic  miles  of  water,  enough  to  cover  all  New 
England  more  than  half  a  mile  deep.  As  indicated  on  page 
107,  a  portion  of  this  rain  water  sinks  into  the  ground, 
a  portion  runs  off  over  the  surface,  and  a  third  part  is  evapo- 
rated. The  immediate  run-off,  reenforced  by  the  overflow 
of  lakes,  by  contributions  from  springs  and  seepage  and  from 
melting  snows,  flows  always  from  higher  to  lower  levels. 
All  that  is  not  lost  by  evaporation  or  by  sinking  beneath 
the  surface  therefore  runs  ultimately  to  the  sea.  Streams 
are  estimated  to  carry  6500  cubic  miles  of  water  (enough 
to  cover  Connecticut  and  Rhode  Island  more  than  a 
mile  deep)  to  the  sea  each  year.  This  water  descends  on 
the  average  nearly  half  a  mile  before  reaching  the  ocean. 
During  the  descent  a  large  but  unknown  amount  of  energy 
is  used  up  as  internal  friction  in  flowing  on  low  slopes,  and 
as  friction  on  the  channels  of  the  streams  and  on  the  sedi- 
ment which  the  streams  carry.  In  addition,  an  enormous 
amount  of  energy  is  exerted  in  geological  work.  The  nature 
of  this  work  and  the  results  are  discussed  below. 

THE  PROCESSES  OF  EROSION 

The  general  term  erosion  covers  those  processes  by  which 
rock  surfaces  are  worn  and  broken  up,  and  the  loosened 
material  removed.  It  therefore  includes  weathering,  trans- 
portation, and  corrasion.  By  the  last  is  meant  the  mechani- 
cal wearing  of  rocks,  particularly  by  running  water, 

125 


126 


PHYSICAL  GEOLOGY 


WEATHERING 

Processes  of  weathering.  —  As  earlier  discussions  have 
indicated,  weathering  is  a  term  applied  to  nearly  all  the 
processes  which  cause  rocks  to  break  up  and  decay.  From 
the  standpoint  of  general  erosion,  weathering  may  be  denned 
as  preparation  for  transportation,  for  it  reduces  rocks  to 

pieces  sufficiently  small 
to  be  blown  or  washed 
away.  Furthermore, 
most  of  the  material 
moved  by  wind  and  Water 
was  derived  from  bed 
rock  in  this  way.  The 
mantle  rock  and  materi- 
als dissolved  in  the  hy- 
drosphere are  the  most 
important  products  of 
weathering.  The  princi- 
pal processes  of  weather- 
ing were  discussed  in 
connection  with  the  work 
of  the  atmosphere  and 
of  ground  water  (pp. 
100-104, 117).  Some  of  the  remaining  processes  may  be  noted 
here.  The  mechanical  beating  of  raindrops  disturbs  small  sur- 
face particles  and  causes  them  to  wear  one  another  to  slight 
extent.  More  important  is  the  work  done  by  plants  and  ani- 
mals. The  growth  of  roots  in  joints  and  other  cracks  of  the 
rocks  enlarges  the  openings,  and  by  so  doing  not  only  helps 
directly  to  break  the  rocks,  but  increases  the  surface  exposed 
to  other  weathering  agents.  Root  splitting  is  illustrated  by 
Figure  116.  Burrowing  animals  make  openings  in  the  rocks, 
and  in  the  aggregate  bring  large  quantities  of  material  to  the 
surface,  where  it  is  exposed  to  the  attack  of  the  weather. 
Gravity  is  regarded  usually  as  an  agent  of  weathering.  For 


FIG.  116.  —  A  tree  growing  in  an  opening 
in  a  rock.  The  growth  of  the  tree  has 
pried  the  parts  of  the  rock  apart,  and 
enlarged  the  opening  from  a  narrow 
crack  to  its  present  size.  Lansing, 
Mich.  (Macpherson.) 


THE  WORK   OF  STREAMS 


127 


FIG.  117.  —  An  example  of  unequal  weathering.  Granite  bowlders  have 
weathered  out  of  an  easily  disintegrated  formation.  Just  south  of  the 
Tropic  of  Cancer,  on  the  Asiatic  mainland  opposite  Hong  Kong.  (R.  T. 
Chamberlin.) 

the  most  part,  however,  it  merely  moves  material  already  pre- 
pared for  transportation  by  other  agents  down  to  lower  levels. 
Rate  of  weathering.  —  The  rate  at  which  weathering  pre- 
pares material 
for  removal 
varies  greatly 
with  (1)  the  char- 
acter of  the  rock 

(Fig.     117),    (2)  „  . 

the  climate,  and      UHPjKflL  >^     % 

(3)  the  rate  at  IliiQBJHKtA " 
which  the  waste 
already  formed  is 
removed.  (1) 
Open-textured 
rocks  with  many 

inir»t«    anrl    rvrViPr      FlG<    1^.  —  St.    Peters    Sandstone,    eastern    Iowa. 
J01  Showing  effects  of  joints  and  bedding  planes  upon 

cracks    absorb         weathering. 


128 


PHYSICAL  GEOLOGY 


much  water,  and  so  favor  the  wedge  work  of  ice,  and,  where 
some  of  the  constituents  of  the  rock  are  soluble,  solution. 

Figures  118  and  119  illus- 
trate the  influence  of  j  oints 
upon  the  weathering  of 
stratified  rocks,  and  Fig- 
ure 120  shows  the  effect  of 
joints  upon  the  weather- 
ing of  granite.  Dark  ob- 
jects heat  and  cool  more 
rapidly  than  light  ones, 
and  dark  rocks  accord- 
ingly favor  splitting 
through  changes  in  tem- 
perature. (2)  No  single 
type  of  climate  favors  all 
the  processes  of  weather- 
ing. It  was  seen  on  page 
100  that  the  wedge  work 
of  ice  is  most  important 
in  moist  regions  where 
there  are  frequent  changes  in  temperature  which  involve  the 
freezing  point.  The  chemical  work  of  the  atmosphere 
and  of  ground  water  is,  on  the  other  hand,  most  important 
in  hot,  moist  climates.  An  arid  climate  with  great  daily 
range  in  temperature  favors  rock  splitting,  but  opposes  the 
work  of  plants,  animals,  and  ground  water.  If  rocks  are 
not  buried  too  deeply  with  soil  and  subsoil,  they  probably 
weather  fastest,  everything  considered,  in  a  hot  and  moist 
climate;  but  flat  lands  in  regions  having  such  climates 
usually  have  thick  accumulations  of  mantle  rock.  It  is 
said  to  reach  a  thickness  of  300  feet  or  more  in  parts  of 
Brazil.  (3)  If  the  products  of  weathering  remain  where 
formed,  they  finally  cover  the  bed  rock  so  deeply  that  it 
is  more  or  less  completely  protected  from  further  attack. 
If,  on  the  other  hand,  they  are  removed  as  fast  as 


FIG.  119.  —  Limestone  columns  weath- 
ering away.  The  openings  between  the 
columns  are  enlarged  joints.  The  sur- 
rounding rock  has  been  removed  by  ero- 
sion. Eastern  Iowa. 

What  are  the  various  ways  in  which 
these  rocks  are  being  weathered  ?  How 
may  the  preservation  of  these  rocks 
after  the  removal  of  the  surrounding 
rocks  be  explained  ? 


THE  WORK  OF  STREAMS 


129 


formed,  so  that  bare  rock  is  always  exposed,  the  work  of 
ground  water  and  of  plants  and  animals  is  reduced  greatly. 
It  consequently  follows  that,  other  things  equal,  weathering 


FIG.  120.  —  Weathered  forms  in  granite,  Laramie  Hills,  Wyo.  Three  sets 
of  joints  may  be  seen,  and  their  influence  upon  the  weathering  of  the 
rock  is  clearly  evident. 

proceeds  most  rapidly  when  its  products  are  rather  promptly, 
but  only  partially,  removed. 

The  fact  that  over  most  of  the  surface  of  the  land  there 
is  a  mantle  of  soil  and  subsoil  indicates  that,  in  general, 
weathering  exceeds  transportation. 


Questions 

1.  Does  the  absence  of  soil  in  any  given  place  mean  that  weather- 
ing is  not  in  progress  there  ? 

2.  What  are  the  principal  agents  of  weathering  in  the  Sahara? 
New  York  ?     Louisiana  ?     The  Amazon  Valley  ? 

3.  Would  a  given  stone  wall  stand  longer  in  Labrador  or  in 
Florida  ?     What  are  the  principal  agents  of  weathering  by  which 
it  would  be  destroyed  ultimately  in  each  place? 

4.  What  differences  in  weathering  might  reasonably  be  expected 
on  the  two  sides  of  an  east  and  west  valley  ?     On  the  two  sides  of  a 
high  north  and  south  mountain  range  in  the  latitude  of  the  United 
States  ?     Of  an  east  and  west  range  ? 


130  PHYSICAL  GEOLOGY 

5.  Flint  nodules  are  of  common  occurrence  in  limestone  (p.  121). 
Explain  the  fact  that  in  certain  limestone  regions  the  stream  beds 
contain  few  limestone  bowlders,  but  many  of  flint. 

6.  Why  does  residual  mantle  rock  in  many  cases  merge  gradually 
into  the  firm  rock  beneath?     (See  Fig.  214.) 

7.  Describe  and  explain  what  you  see  in  Figure  121. 


FIG.  121.  —  Granite  rocks.     Laramie  Hills,  Wyo. 


TRANSPORTATION   BY   STREAMS 

Getting  a  load.  —  Streams  roll  and  drag  material  along 
their  beds,  and  carry  it  in  mechanical  suspension  and  in 
solution.  That  which  is  moved  mechanically  is  obtained 
in  a  variety  of  ways.  Streams  wear  material  from  their 
beds  and  banks,  and  remove  that  which  is  already  loose. 
Sediment  is  brought  in  by  tributaries.  Material  loosened 
by  weathering  on  the  tributary  slopes  is  delivered  to  the 
stream  by  gravity  or  by  rain  wash.  A  certain  amount  of 
fine  material  is  brought  by  the  wind.  Most  of  the  material 
which  is  carried  in  solution  is  contributed  by  issuing  ground 
waters;  a  small  part  is  furnished  by  the  unorganized  run- 
off, and  another  minor  portion  is  dissolved  by  streams  from 
the  rocks  over  which  they  flow. 

How  the  load  is  carried. — -A  stream  pushes  sediment  along 
its  bottom  by  the  direct  impact  of  the  current,  and  also 
rolls  and  drags  it  by  the  friction  of  the  bottom  water,  some- 


THE  WORK  OF  STREAMS 


131 


— C 


what  as  one  might  move  sand  grains  on  a  table  by  dragging 
the  outstretched  hand  across  them.  Material  is  held  in 
suspension  chiefly  by  minor  up-moving  currents.  Since 
rock  material  is  on  the  average  two  and  one  half  to  three 
times  as  heavy  as  the  water  it  displaces,  it  tends,  under 
gravity,  to  sink  to  the  bottom.  In  standing  water  it  sinks 
vertically.  In  a  stream  whose  water  is  moving  horizontally, 
two  forces  act  upon  it.  Gravity,  of  course,  seeks  to  draw  it 
directly  to  the  bottom  (G,  Fig.  122),  while  the  current  tends 
to  move  it  in  the  direction  of  its  flow  (C, 
Fig.  122).  The  sediment  accordingly  fol- 
lows a  course  (S,  Fig.  122),  which  is  a  re- 
sultant of  the  combined  forces.  It  reaches 
the  bottom  in  the'  same  time  it  would  in 
standing  water  of  the  same  depth.  In  na- 
ture, however,  stream  water  rarely,  if  ever, 
moves  horizontally  for  any  distance.  Bowl- 
ders and  other  irregularities  on  the  bottom 
deflect  portions  of  the  main  current  ob- 
liquely upwards.  Projections  of  the  bank 
likewise  create  subordinate  currents,  some 
of  which  move  upwards.  Sediment  settling 
to  the  bottom  along  an  oblique  path  may 
encounter  such  up-going  currents  and  be 
lifted  by  them.  Presently  sinking  again,  it 
may  again  be  lifted  or  may  reach  the  bot- 
tom, perhaps  to  be  presently  carried  up  once  more  by  other 
upward  currents.  Material  the  size  of  sand,  and  larger,  prob- 
ably rarely  makes  extended  trips  in  suspension ;  instead,  many 
short  trips  are  interrupted  by  periods  when  it  rests  upon  the 
bottom,  or  is  dragged  and  rolled  along  it.  On  the  other  hand, 
mud  and  silt  are  often  carried  long  distances  before  settling. 
Nevertheless,  even  fine  material  probably  normally  requires 
thousands  of  years  to  make  the  trip  from  the  sources  of  the 
larger  rivers  to  their  mouths,  for  it  is  dropped  on  the  way 
many  times  for  long  periods,  perhaps  helping  to  form  bars  and 


FIG.  122.  —  Diagram 
showing  the  two 
forces  acting  upon 
a  particle  at  A  in 
the  horizontal  cur- 
rent of  a  stream, 
and  the  general 
course  which  the 
particle  takes  in 
sinking  to  the  bot- 
tom. 


132  PHYSICAL  GEOLOGY 

islands  or  being  built  into  flood  plains,  from  which  it  is  re- 
moved later,  to  be  carried  another  stage  on  its  journey  to  the 
sea.  Just  as  in  the  atmosphere  (p.  87),  material  sinks  more 
slowly  in  proportion  as  the  surface  it  exposes  to  the  friction 
of  the  water  is  great  in  relation  to  its  weight.  Sediment  of  a 
given  sort  settles  faster  in  salt  water  than  in  fresh  water. 

The  amount  of  the  load.  —  The  amount  of  the  load  which 
any  given  stream  is  moving  depends  upon  (1)  its  velocity, 
(2)  its  volume,  and  (3)  the  amount  and  nature  of  the  ma- 
terial to  which  it  has  access.  Obviously,  the  swifter  and 
larger  a  stream,  the  more  and  larger  the  material  it  is  capable 
of  moving.  But  many  swift  streams  carry  little  material, 
because  little  loose  material  is  available,  or  because  it  is  in 
pieces  too  large  to  be  moved.  The  Mississippi  River  carries 
on  the  average  over  1,000,000  tons  of  sediment  per  day  into 
the  Gulf  of  Mexico.  It  has  been  calculated  that  it  dis- 
charges sediment  sufficient  to  fill  the  basin  of  Lake  Superior, 
the  largest  lake  in  the  world,  with  an  area  of  32,000  square 
miles  and  an  average  depth  of  550  feet,  in  about  66,000 
years.  It  has  been  estimated,  also,  that  the  work  performed 
each  year  by  the  Missouri  River  in  transporting  sediment 
is  equivalent  to  275,000,000,000  mile  tons,  or  tons  carried 
one  mile.  The  railroads  of  the  United  States  carried 
236,600,000,000  mile  tons  in  1907. 

Questions 

1.  Why  are  many  mountain  streams  clear? 

2.  Why  in  many  streams  are  narrows  in  the  channel  floored  with 
coarse  material,  while  broad  parts  of  the  channel  are  lined  with  fine 
sediment  ? 

3.  Do  two  streams  of  the  same  velocity  and  volume  necessarily 
carry  the  same  amount  of  sediment?     Reasons? 

4.  Can  a  given  stream  carry  a  greater  weight  of  coarse  or  of  fine 
material  ?     Why  ? 

5.  (1)  Just  why  do  streams  carry  more  sediment  after  heavy 
rains  ?    (2)  Would  the  effect  of  a  given  rain  in  northern  United  States 
be  likely  to  be  the  same  in  January  as  in  July  ?    (3)  Make  a  general 
statement  concerning  erosion,  in  keeping  with  the  answer  to  (2). 


THE  WORK  OF  STREAMS  133 

COBRASION 

How  streams  wear  rock.  —  Like  clear  air,  clear  water  can 
do  little  in  the  way  of  mechanically  wearing  firm  rocks.  Per- 
haps the  most  striking  illustration  of  this  is  afforded  at 
Niagara  Falls.  Seven  thousand  tons  of  essentially  clear  water 
rush  over  the  brink  of  the  falls  each  second,  and  yet  certain 


FIG.  123.  —  The  tools  of  a  river.    Stream-worn  pebbles  in  the  bed  of  the 
Potomac  River  at  Barnum,  Md.     (Md.  Geol.  8urv.) 

tiny  plants  grow  in  the  water,  clinging  to  the  rocks  at  the 
very  edge.  Were  erosion  actively  in  progress  at  the  edge,  the 
plants  would,  of  course,  be  swept  away.  The  St.  Lawrence 
River  leaves  Lake  Ontario  as  clear  as  the  lake  waters  them- 
selves, and  for  many  miles  is  unable  to  corrade  effectively, 
even  where  its  current  has  great  velocity  and  washes  the  shores 
of  islands  whose  banks  are  of  clay.  Many  other  streams 
which  flow  from  lakes  illustrate  the  same  thing.  They  often 
have  mossy  channels  in  spite  of  swift  currents.  Streams,  like 
winds,  wear  rocks  by  means  of  the  rock  fragments  which  they 
transport  (Fig.  123).  Sand  grains,  pebbles,  etc.,  that  are 
swept  along  by  the  main  current,  rub,  rasp,  and  strike  the  bed 
and  sides,  breaking  and  wearing  pieces  from  them.  Material 
B.  &  B.  GEOL.  — 9 


134  PHYSICAL  GEOLOGY 

in  suspension  is  also  frequently  driven  vigorously  against  the 
bottom  by  subordinate  downward-moving  currents,  with 
similar  effect.  The  tools  are  themselves  worn  in  the  process. 
Stream-swept  stones  become  rounded  (Fig.  123),  and  their 
surfaces  often  have  many  tiny  pits,  or  depressions,  made  by 
the  blows  they  have  delivered  or  received.  These  characteris- 
tics have  helped  to  prove  that  the  material  of  certain  rock 
formations  was  handled  by  vigorous  streams. 

Rate  of  wear.  —  The  rate  at  which  degrading  streams  lower 
their  channels  depends  on  several  conditions.  (1)  Weak 
rocks  with  soluble  cements  favor  rapid  wear,  while  strong, 
nonsoluble  rocks  retard  it.  Stratified  rocks  in  general  prove 
less  resistant  than  massive  rocks.  Other  things  being  equal, 
rocks  with  numerous  joints  and  cracks  are  worn  faster 
than  others,  because  these  openings  are  planes  of  weakness. 
(2)  Rapid  streams  deal  harder  blows  and  more  of  them  than 
slow  ones,  and  so,  other  things  being  equal,  wear  their  Channels 
faster.  The  velocity  of  a  stream,  in  turn,  depends  upon  (a) 
the  slope  (gradient}  of  its  channel,  (6)  its  volume,  (c)  its 
load,  and  (d)  the  shape  of  its  channel.  Obviously,  the 
steeper  the  channel  and  the  larger  the  stream,  the  greater  its 
velocity.  Energy  is  expended  in  moving  sediment,  which 
otherwise  would  express  itself  in  greater  velocity ;  other  things 
equal,  a  given  stream  accordingly  flows  fastest  if  clear,  and 
slowest  if  loaded.  A  stream  is  retarded  by  friction  with  its 
bed  and  sides.  Crooked  channels,  with  wide,  uneven  bottoms, 
occasion  great  friction,  and  tend  to  produce  a  sluggish  cur- 
rent ;  straight  channels  with  narrow  and  smooth  bottoms  de- 
velop less  friction,  and  promote  greater  velocity.  (3)  Since 
the  velocity  of  a  stream  is  decreased  as  its  load  is  increased, 
it  follows  that  the  force  of  its  blows  is  also  diminished.  In 
other  words,  the  greater  the  number  of  tools  carried,  the 
greater  the  number  of  blows  delivered  in  a  given  time,  but 
the  weaker  each  blow  is ;  while,  on  the  other  hand,  the  fewer 
the  tools  carried,  the  fewer  the  blows  delivered  in  a  given  time, 
but  the  stronger  each  blow  becomes.  Clearly,  streams  wear 


THE  WORK  OF  STREAMS  135 

fastest,  other  things  being  equal,  when  carrying  a  partial  load, 
so  that  many  blows  are  delivered,  but  not  so  many  that  all  are 
weak.  (What  qualities  should  render  rock  fragments  most 
efficient  tools  for  stream  corrasion?  Would  tools  possessing 
these  qualities  long  retain  them  all?  Why?) 

Graded  streams.  —  When  the  gradient  of  a  stream  is  just 
steep  enough  to  give  it  the  velocity  necessary  to  wash  forward 
the  sediment  brought  to  it  from  the  tributary  slopes,  it  is  said 
to  be  at  grade.  If  it  is  able  to  transport  more  than  is  delivered, 
it  removes  material  from  its  bed  until  it  comes  to  grade  at  a 
lower  and  gentler  slope.  If  it  is  unable  to  transport  all  that 
is  delivered,  part  of  the  load  is  left  as  a  deposit.  By  this 
means  the  channel  is  raised  and  the  gradient  becomes  steeper 
gradually,  until  in  time  the  stream  grows  swift  enough  to  carry 
away  the  sediment  brought  to  it. 

Rate  of  land  reduction  by  stream  erosion.  —  Estimates 
have  been  made  of  the  rate  at  which  certain  river  systems  are 
degrading  their  basins.  This  may  be  done  as  follows :  The 
width,  average  depth,  and  mean  velocity  of  the  main  river 
at  its  mouth  may  be  determined  at  different  times  by  measure- 
ments, and  from  these  data  the  average  volume  of  water  dis- 
charged per  year  may  be  calculated.  The  average  amount 
of  material  contained  in  a  cubic  foot  of  the  water,  both  in  solid 
form  and  in  solution,  may  also  be  learned  by  examination  of 
numerous  samples.  Knowing  the  average  amount  of  sedi- 
ment in  each  cubic  foot  of  the  water,  and  the  average  number 
of  cubic  feet  discharged  in  a  year,  the  total  amount  of  sediment 
delivered  at  the  mouth  of  the  river  may  be  computed  readily. 
Finally,  the  area  of  the  drainage  basin  being  known,  one  may 
determine  to  what  uniform  depth  the  sediment  removed 
yearly  would  cover  it.  The  result  indicates  the  average  rate 
per  year  at  which  the  drainage  basin  is  being  degraded.  By 
this  general  method  it  has  been  estimated,  for  example,  that 
the  Mississippi  Basin  is  being  lowered  mechanically  at  the 
average  rate  of  one  foot  in  about  5000  years,  and  when  the 
amount  removed  in  solution  is  considered  also,  one  foot  in  3500 


136  PHYSICAL  GEOLOGY 

years.  The  Ganges  Basin  is  being  reduced  by  the  removal  of 
material  in  the  solid  form  alone,  at  the  rate  of  a  foot  in  less 
than  2000  years ;  and  the  Danube  Basin  a  foot  in  approxi- 
mately 6800  years.  In  some  parts  of  each  basin  the  rate  is  far 
more  rapid.  Figure  124  shows  the  results  of  recent  estimates 
of  the  rate  of  land  reduction  throughout  the  United  States. 


FIG.  124.  —  Rates  of  land  reduction  by  stream  erosion  in  the  United  States. 
The  figures  are  the  number  of  years  required  for  one  inch  of  denudation. 
(After  National  Conservation  Commission.) 

The  average  elevation  of  the  continents  above  sea  level  is 
about  2300  feet.  If  they  were  being  degraded  by  streams  at 
the  average  rate  of  the  Mississippi  Basin,  and  continued  to  be 
cut  down  at  that  rate  to  sea  level,  and  nothing  occurred -to  off- 
set the  work  of  the  streams,  the  lands  would  be  destroyed  in 
about  8,000,000  years.  Probably,  however,  the  average 
rate  of  land  reduction  is  less  than  that  assumed.  Nor  could 
the  present  rate,  whatever  it  may  be,  continue  till  the  land 
was  at  sea  level,  for  as  it  gets  lower,  the  streams  would  flow 
more  slowly,  and  therefore  degrade  less  rapidly.  Again,  judg- 
ing by  the  past,  diastrophism  and  vulcanism  would  intervene 
to  maintain  the  land  masses.  Still  other  factors  would  modify 


THE  WORK  OF  STREAMS  137 

the  problem,  as,  for  example,  the  fact  that  streams  cannot  re- 
duce their  basins  quite  to  sea  level  (p.  139)  and  that  an  average 
of  2300  feet  would  not  have  to  be  eroded  away  to  bring  the 
land  to  the  level  of  the  sea,  for  the  surface  of  the  ocean  would 
be  raised  by  the  deposition  in  it  of  the  waste  from  the  land. 
Nevertheless,  such  computations  are  worth  while,  since  they 
aid  one  to  appreciate  the  importance  of  the  work  being  done 
by  running  water. 

Questions 

1.  Why  is  the  Niagara  River  practically  free  from  sediment? 

2.  Other  things  being  equal,  would  a  given  stream  corrade  faster 
when  flowing  across  the  edges  of  highly  tilted  beds,  or  on  horizontal 
beds  ?    Why  ?    When  the  beds  dip  downstream  or  upstream  ?   Why  ? 

3.  Is  corrasion  favored  more  by  a  constant  volume,  or  by  great 
and  sudden  fluctuations  in  a  stream  ? 

4.  Enumerate  all  the  conditions  which  might  enable  one  of  two 
streams  of  equal  and  constant  volume  to  corrade  much  faster  than 
the  other. 

5.  Is  it  possible  for  a  stream  to  corrade  without  degrading  ?     To 
degrade  without  corrading  ? 

6.  Will  a  given  stream  flow  faster  when  fully  loaded  with  coarse 
or  with  fine  material  ? 

FEATURES  DEVELOPED  BY  RIVER  EROSION 
VALLEYS 

Most  streams  flow  in  valleys.  In  general,  valleys  correspond 
in  size  to  their  streams,  and,  like  the  stream  it  contains,  a 
given  valley  is  smaller  than  the  one  it  joins,  and  larger  than 
those  which  join  it.  At  their  union,  the  bottoms  of  tributary 
valleys  are  normally  at  the  same  level  as  the  bottoms  of  the 
larger  valleys  to  which  they  lead.  Furthermore,  all  streams 
are  engaged,  with  the  help  of  the  agents  of  weathering,  in  en- 
larging their  valleys.  These  facts  indicate  that  the  valleys 
were  not  found  ready-made  by  the  streams  which  occupy  them, 
but  that  they  are  a  result  of  the  work  of  the  streams  aided  by 
weathering  agents.  Many  synclinal  troughs  (p.  66)  form 
valleylike  depressions.  Since  they  are  due  to  the  structure 


138 


PHYSICAL  GEOLOGY 


of  the  rocks,  such  valleys  are  called  structural  valleys.     They 
usually  contain  stream  valleys  in  their  bottoms. 


FIG.  125.  —  Sketch  of  a  gully. 


FIG.  126.  —  Gullies  near  River- 
side, Cal.     (Fairbanks.) 

the  gully  will  make  it 
longer ;  water  coming  over 
the  sides  will  make  it 
wider ;  and  the  water  which 
flows  along  the  bottom 
will  make  it  deeper.  Thus 
it  may  become  sufficiently 
long  and  wide  and  deep  to 
be  called  a  ravine,  and 
finally  a  valley.  When  its 
bottom  is  worn  below  the 


The  beginning  of  a 
valley.  —  Figure  125 
shows  an  infant  valley 
or  gully,  which  con- 
tains running  water 
only  during  rains.  In 
the  future,  rain  water 
running  down  the 
slope  into  the  head  of 


FIG.     127.  —  A    mountain    ravine    near 
Marshall,  N.  C.     (U.S.  Geol.  Surv.) 


THE  WORK  OF  STREAMS  139 

water  table,  ground  water  will  enter  it  as  seepage  and  springs 
from  the  sides,  and  flow  away  as  a  stream.  When  the  bottom 
of  a  valley  is  below  the  wet-weather  level  of  the  water  table, 
but  above  the  dry-weather  level,  it  contains  an  intermittent 
stream,  but  when  the  bottom  of  the  valley  is  eroded  below  the 
water  table  at  its  lowest  level,  the  stream  is  permanent,  and 
the  enlargement  of  the  valley  proceeds  without  interruption. 
Figure  126  shows  many  gullies  starting  on  an  unprotected  sur- 
face in  a  relatively  dry  region,  while  Figure  127  shows  a 
mountain  ravine  in  a  humid  region. 

Valley  deepening.  —  A  stream  lowers  its  channel,  and  so 
deepens  its  valley,  by  removing  material  loosened  by  weather- 


FIG.   128.  —  A  stream  undercutting  its  bank  and  widening  its  valley. 
Central  Illinois.     (Crane.) 

ing  or  by  its  own  corrasion.  But  there  is  a  limit  below  which 
a  stream  cannot  degrade  its  valley  flat.  This  is  the  level  of  the 
lake,  sea,  or  other  valley  to  which  it  leads.  Furthermore,  it 
can  cut  to  this  level  only  at  its  mouth,  from  which  the  valley 
bottom  rises  upstream,  very  gently  in  the  case  of  large 
rivers,  and  more  rapidly  where  the  stream  is  small.  For  some 
distance  above  their  mouths,  streams  may,  however,  cut  their 
channels  slightly  below  the  level  of  the  sea  or  lake  into  which 
they  flow.  The  lowest  level  to  which  a  stream  can  cut  is 


140 


PHYSICAL  GEOLOGY 


base  level.     As  a  stream  approaches  base  level,  it  flows  on  a  di- 
minishing slope,  and  its  current  therefore  becomes  less  and  less 

rapid.  In  other  words,  a 
stream  approaches  .base  level 
more  and  more  slowly  as  it 
draws  nearer  and  nearer  to  it, 
so  that  the  removal  of  the  last 
few  feet  may  take  longer  than 
all  the  rest. 

Valley  widening.  —  Valleys 
are  widened  in  a  variety  of 
ways.  Relatively  sluggish 
streams  are  pushed  aside  by 
the  currents  of  their  tribu- 
taries or  by  obstacles.  In 
this  way  the  stream  is  driven 
first  against  one  bank,  and 
then  against  the  other,  and 
so  undermines  each.  The 
points  of  attack  varying  from 
time  to  time,  the  valley  is 
opened  generally,  and  a  val- 
ley flat  is  developed  (Figs.  128 
and  129).  Meanwhile,  other 
agencies  assist  in  widening 
the  valley.  Rains  wash  weathered  material  down  its  sides, 
and  if  the  slopes  are  sufficiently  steep,  fragments  also  roll  and 
fall  down  them.  Material  works  its  way  down  the  sides  of 
the  valley  also  by 
creep  and  by  slump- 
ing (p.  115),  and  is  re- 
moved in  other  less 
important  ways.  If 

the     material     re-    FIG.  130.  —  The  divide  between  the  two  valleys 

mained  at   the   hot-      is  being  Tconsum^  by  the  side  cutting  of  the 

rivers.     It  may  be  cut  away  entirely,  in  which 
tom,  the  enect  WOUld        case  the  two  valleys  will  become  one. 


FIG.  129.  —  Diagrams   of  a  river  de- 
veloping a  flat  by  side  cutting. 


THE  WORK  OF  STREAMS 


141 


be  to  narrow  the  valley  there,  while  widening  it  above.  Us- 
ually, however,  it  is  carried  away  by  the  stream.  By  the 
widening  of  two  adjacent  valleys,  the  intervening  ridge  may 
be  worn  out,  the  two  becoming  one  (Fig.  130).  The  contin- 
uation of  this  process  among  neighboring  valleys  would  ulti- 
mately reduce  the  entire  surface  of  the  area  affected  to  the 
level  of  the  valley  bottoms. 

Valley  lengthening.  —  The  heads  of  valleys  are  usually 
without  permanent  streams,  for  they  are  commonly  above 
the  lowest  level  of  the 
ground-water  surface. 
The  stream,  therefore, 
does  not  assist  in  the 
lengthening  of  its  valley 
headward,  but  all  the 
other  agencies  which 
widen  valleys  help  also 
to  lengthen  them.  A  valley  ceases  to  grow  by  headward 
erosion  when  a  permanent  divide  (Fig.  131)  is  established. 
This  is  when  the  wear  accomplished  by  the  run-off  which 


a 


FIG.  131.  — Diagram  of  a  divide.  The 
crest  of  the  divide  (at  a)  is  permanent  if 
the  conditions  of  erosion  are  the  same  on 
the  two  sides.  Rainfall  may  lower  it, 
but  it  cannot  shift  its  position  horizon- 
tally. 


FIG.  132.  —  Bad-land  topography  near  Grand  Junction,  Colo.    Shows  many 
gullies.     (Baker.) 

enters  the  head  of  the  valley  is  balanced  by  the  erosion  of  the 
water  which  runs  from  the  divide  in  the  opposite  direction. 
Thus  limits  are  set  to  the  growth  of  a  valley  in  all  three 


142  PHYSICAL  GEOLOGY 

dimensions.  In  depth,  the  limit  is  base  level ;  in  width  and 
length  it  is  fixed  by  neighboring  valleys. 

Struggle  among  valleys.  —  It  is  not  to  be  inferred  from 
what  has  preceded  that  all  gullies  become  valleys,  or  even 
ravines.  Quite  the  opposite  is  true.  Few  of  the  gullies  shown 
in  Figure  132,  for  example,  can  grow  to  ravinehood.  As 
they  widen,  the  intervening  divides  will  be  worn  out,  combin- 
ing adjacent  gullies  and  reducing  the  number.  Many  gullies 
are  commonly  destroyed  in  the  formation  of  a  single  ravine, 
which  in  turn  is  likely  to  presently  find  its  growth  contested  by 
other  ravines.  Such  a  conflict  is  shown  in  Plate  II,  among  the 
ravines  near  Wesley.  Little  opportunity  for  growth  remains 
to  most  of  the  ravines  in  the  vicinity,  and  many  are  doomed 
to  early  destruction  by  their  more  powerful  neighbors. 

Valleys  without  gullies.  —  Not  all  valleys  have  grown  from 
gullies  as  described  above.  In  the  northern  part  of  the  United 
States  and  in  Canada,  for  example,  thousands  of  lakes  were 
formed  during  the  Glacial  period  (p.  214).  In  the  moist  cli- 
mate of  this  region,  the  lakes  received  more  water  as  rain  on 
their  surfaces  and  as  run-off  from  tributary  slopes,  than  they 
lost  by  evaporation.  Consequently,  many  overflowed  their 
rims,  forming  streams.  Such  streams  followed  the  lowest 
available  lines  of  descent  to  other  streams  or  lakes,  and  by 
erosion  developed  valleys.  Thus  the  streams  existed  before 
the  valleys.  In  this  and  other  ways,  streams  and  valleys  of 
this  class  are  in  contrast  with  those  considered  first. 

Tributary  valleys.  —  Most  valleys  have  tributaries,  and 
these  in  turn  branch  repeatedly,  like  the  limbs  of  a  tree.  A 
main  valley  and  all  its  tributary  valleys  constitute  a  valley 
system,  whose  streams,  the  main  river  and  all  its  branches, 
form  a  river  system.  The  entire  area  drained  by  a  river  system 
is  a  drainage  basin.  Tributary  valleys  commonly  start  as 
gullies  on  the  sides  of  their  parent  valleys.  If  the  slope  of 
the  ground  back  from  the  sides  of  a  valley  is  such  that  more 
water  enters  it  at  some  points  than  at  others,  the  velocity, 
and  hence  the  erosive  power,  of  the  entering  water  will  be 


THE  WORK  OF  STREAMS 


143 


greater  at  such  places  than  elsewhere,  and  tributary  depres- 
sions (gullies)  will  result.     Even  if  an  equal  amount  of  water 

entered    the    parent   valley   at    all 

points,  the  same  result  would  follow, 

provided  the  rocks  of  the  valley  sides 

were  of  unequal  strength,  for  erosion 

would  be  more  rapid  where  the  rocks 

were  weaker,  giving  rise  to  tributary   FlG 

depressions. 

Stages  in  topographic  development.  —  It  is  apparent  from 

the  preceding  paragraphs  that  valleys  pass  through  careers 

just  as  men  do. 
Each  stage  in  the 
career  of  a  valley 
is  characterized  by 
certain  features, 


133.  —  Cross  section  of 
a  young  valley. 


FIG.  134.  —  Cross  section  of  a  mature  valley. 


FIG.  135.  —  Cross  section  of  an  old  valley. 


so  that  by  observ- 
ing the  form  of  a 

given  valley,  one  may  determine  readily  what  point  it  has 
reached  in  its  development.  Valleys  are  young  when  still 
narrow  and  steep - 
sided  (Fig.  133). 
These  features  in- 
dicate that  as  yet 
down  cutting  is  keeping  ahead  of  other  processes.  Most  young 

valleys  have  few  and  im- 
perfectly developed  trib- 
utaries and  relatively 
steep  gradients.  (What 


things  will  determine 

FIG.  136.  —  Diagram   showing  changes  in  whether     Or     not     young 

the  shape  of  a  valley  as  it  advances  from  vfliipvc,  flrp  rippn  <?\   Yol]r,n- 

youth  to  old  age.     The  material  in  which  valleYs  are  de 

the  valley  is  cut  is  all  of  the  same  char-  valleys   in  various  stages 

acter-  of    development     are 

shown  on  Plate  II.     Mature  valleys  are  wider,  deeper,  and 
have  gentler  gradients  and  more  and  larger  tributaries.     In 


PLATE  III.  AN  AREA  IN  A  YOUTHFUL  STAGE  OF  EROSION.  Contour 
interval,  10  feet.  Scale,  about  1  mile  per  inch.  (Bowling  Green,  Ohio, 
Sheet,  U.  '  S.  Geological  Survey.) 


146 


PHYSICAL  GEOLOGY 


early  maturity  they  are  roughly  U-shaped  (Fig.  134),  instead 
of  V-shaped,  as  before.  In  later  maturity  they  have  conspicu- 
ous flats.  These  changes  in  cross  section  signify  that  down 


FIG.  137.  —  Diagram  of  an  area  in  a  youthful  stage  of  erosion.  The  area  is 
situated  some  distance  from  the  sea.  The  bottom  of  the  diagram  repre- 
sents sea  level. 


FIG.  138.  —  Diagram  showing  mature  topography  in  a  region  situated  some 
distance  from  the  sea.  The  bottom  of  the  diagram  is  sea  level.  The 
area  shown  in  Figure  137  will  in  time  closely  resemble  the  present  appear- 
ance of  this  area. 


FIG.  139.  —  Diagram  showing  old  topography  in  a  region  situated  some 
distance  from  the  sea.  The  bottom  of  the  diagram  is  ssa  level.  Unless 
diastrophism  interferes  with  the  work  of  the  streams,  the  areas  represented 
in  the  two  preceding  figures  will  finally  closely  resemble  this  area. 

cutting  has  come  to  be  very  slow,  and  that  the  processes  which 
widen  valleys  and  reduce  their  sides  to  gentler  slopes  have 
become  much  more  important,  relatively.  Valleys  are  old 


THE  WORK  OF  STREAMS  147 

when  their  nearly  level,  wide  bottoms  are  bounded  by  low, 
gently  sloping  sides  (Fig.  135).  Figure  136  shows  the  chang- 
ing shape  of  a  valley  in  its  advance  from  youth  to  old  age. 

The  terms  youth,  maturity,  and  old  age  are  applied  also  to 
rivers  and  to  the  topography  of  drainage  basins.  Figures  137, 
138,  and  139,  and  Plates  III,  IV,  and  V,  show  young,  mature, 
and  old  topographies.  In  Figure  137  the  valleys  are  few  in 
number  and  have  the  characteristics  described  above  as  dis- 
tinctive of  youth.  The  region  is  poorly  drained,  broad  upland 
areas  between  the  valleys  being  as  yet  untouched  by  erosion. 
The  task  of  carrying  to  the  sea  all  the  material  above  base 
level  has  scarcely  begun.  The  area  shown  in  Figure  138  has 
been  eroded  into  a  rough  hill-and-valley  country.  Only  nar- 
row ridges  remain  to  indicate  the  position  of  the  once  broad 
inter-valley  uplands,  while  the  larger  valleys  have  nearly 
reached  base  level.  The  region,  therefore,  possesses  greatest 
relief  at  this  stage.  Slope  is  at  a  maximum,  and  every  part 
of  the  area  is  now  reached  by  drainage  lines.  When  the 
rivers  have  reduced  a  region  to  old  age  (Fig.  139),  it  again 
approaches  flatness  at  a  level  as  low  as  running  water  can 
bring  it.  Such  a  plain,  if  perfected,  would  be  a  base-level 
plain. 

It  is  especially  important  to  note  that  youth,  maturity, 
and  old  age  are  terms  which,  as  used  in  geolog\^  indicate 
stages  in  development  and  not  periods  of  years.  It  is  per- 
fectly possible,  for  example,  for  a  large  river  working  on 
weak  material  to  bring  its  valley  to  old  age  in  the  same  or 
less  time  than  that  required  for  a  smaller  stream,  opposed 
by  stronger  rocks,  to  develop  a  mature  valley. 

Peneplains  and  monadnocks.  —  Areas  have  been  rarely, 
if  ever,  absolutely  base-leveled.  The  time  required  is  very 
great,  and  before  the  rivers  have  accomplished  the  task,  the 
area  is  likely  to  have  been  elevated  with  reference  to  sea 
level  and  the  quickened  streams  started  upon  the  new  task 
of  reducing  it  again.  Extensive  areas  have,  however,  been 
reduced  nearly  to  base  level.  Such  plains  are  called  pene- 


148 


PLATE  IV.     MATURE   TOPOGRAPHY.     Contour  interval,    20  feet.     Scale, 
about  1  mile  per  inch.    (Athalia,  O.-  W.Va.,  Sheet,  U.  S.  Geological  Survey.) 


PLATE  V.    AN  AREA  IN  OLD  AGE.     Contour  interval,  50  feet.      Scale, 
about  2  miles  per  inch.     (Fredonia,  Kan.,  Sheet,  U.  S.  Geological  Survey.) 


150 


PHYSICAL  GEOLOGY 


plains  (almost  plains).     Above  their  otherwise  flattish  sur- 
faces occasional  unreduced  elevations  rise  abruptly.     These 


FIG.  140.  —  A  peneplain  near  Camp  Douglas,  Wisconsin,  with  several  monad- 
nocks  in  the  distance.     (Sankowsky.) 

elevations  are  called  monadnocks  (Figs.  140  and  141),  after 
a  mountain  of  this  type,  in  New  Hampshire,  and  owe  their 
preservation  either  (1)  to  the  superior  resistance  of  their 

rocks,  or  (2)  to  a  favorable  posi- 
tion among  the  streams. 

J    jjr\--^'         /  /  Cycles  of  erosion.  —  A  cycle  of 

"  ^oJl  erosion  is  the  time   required   for 

the  production  of  a  base-level  plain. 
From  the  preceding  paragraph  it  is 
evident  that  erosion  cycles  have 
been  rarely  completed.  Usually 
they  are  interrupted  and  a  new 
cycle  inaugurated. 

The  erosion  history  of  a  region 


FIG.   141.  — A  monadnock  on  is  ofteri  recorded  by  the  character 

the  flood  plain  of  a  river.  „    . ,  ,  .—., 

of  the  topography.  Thus,  mean- 
dering courses  are  developed  by  rivers  only  on  valley  flats,  but 
many  a  meandering  river  occupies  a  valley  scarcely  wider 
than  the  stream  itself,  and  much  narrower  than  the  belt 
within  which  the  river  winds  (Plate  VI).  This  means  that 
after  the  development  of  the  meandering  course,  the  region 
was  elevated  in  such  a  manner  as  to  increase  the  gradient, 


152 


PHYSICAL  GEOLOGY 


and  so  the  velocity,  of  the  stream,  which  was  then  able  to 
cut  a  new  valley  in  the  floor  of  the  old  one.     In  its  growth 

this  young  valley  fol- 
lowed up  the  old  curves 
of  the  river,  and  these 
became  intrenched  me- 
anders. 

Figure  142  shows  a 
young  inner  canon 
formed  in  a  wide  older 
valley  in  consequence  of 
uplift.  After  the  river 
had  developed  a  wide 
flat,  the  valley  was  so 
elevated  as  to  quicken 
(rejuvenate)  the  stream,  enabling  it  to  cut  the  new,  inner 
valley.  The  broad  remnants  of  the  old  valley  flat  constitute 
terraces.  River  terraces  may  be  defined  as  benches  that 


FIG.  142.  —  Sketch  showing  recent  gorge 
in  older  valley.  Matanuska  Valley, 
Alaska. 


FIG.  143.  —  Shoshone  River  at  Cody,  Wyoming.  Shows  Shoshone  Canon 
in  the  distance,  with  the  south  end  of  Rattlesnake  Mountain  on  the  right 
and  Cedar  Mountain  on  the  left,  also  the  nearly  level  surfaces  of  portions 
of  the  successive  terraces.  Irrigation  is  carried  on  extensively  on  the 
terraces.  (Fisher,  U.S.  GeolnSurv.) 


THE  WORK  OF  STREAMS 


153 


extend  along  valleys  and  are  above  the  reach  of  ordinary 
flood  waters  (Fig.  143).  Many  terraces  are  not  due  to  uplift 
(p.  183).  A  rejuvenated 

C 


FIG.  144.  —  Diagram  of  an  interrupted 
profile. 


river  responds  to  the  ele- 
vation first  in  its  lower 
course,  and  the  new  .val- 
ley formed  there  extends 
itself  upstream  by  head- 
ward  erosion.  Until  this 
extension  is  completed, 
the  upper  river,  as  yet 
unaffected  by  the  uplift, 
flows  on  the  relatively 
broad  and  gently  slop- 
ing bottom  of  the  old 
valley  (A.  to  B,  Figs.  144 
and  145),  while  the  lower 
river  flows  in  the  narrow, 
steep-floored  new  valley 
(B  to  C,  Figs.  144  and 
145).  Such  rivers  are 
said  to  have  interrupted 
profiles. 

Figure  146  shows  in  principle  the  topography  and  structure 
of  a  portion  of  the  Appalachian  Mountains.  Water-laid  beds 
(What  indicates  this  origin?)  were  folded  into  a  series  of 


FIG.  145.  —  Diagram  of  a  rejuvenated  area. 
In  what  stage  of  erosion  was  the  re- 
gion before  it  was  rejuvenated  ?  When 
did  the  two  lower  tributary  streams  be- 
gin to  cut  new  valleys?  What  are  all 
the  things  which  may  have  helped  to  de- 
termine the  fact  that  the  new  valley  of 
the  tributary  stream  "D"  is  longer  than 
that  of  "  E  "  ?  What  changes  will  occur  in 
the  character  of  the  topography  in 
the  future  ? 


~~7\~ ^         A 

XSA        D    /ST""? — , 


FIG.  146.  —  Diagram  of  folded  mountains  in  the  youthful  stage  of  the  third 
cycle  of  erosion.     (Modified  after  Salisbury.) 

anticlines  and  synclines,  the  former  constituting  parallel 
mountain  ridges,  the  latter  structural  valleys.  The  area  was 
then  worn  down  to  base  level,  a  broad,  nearly  level  plain  re- 
placing the  mountain  topography.  The  former  existence  of 


154  PHYSICAL  GEOLOGY 

this  base-level  plain  is  indicated  by  the  fact  that  the  present 
ridges  have  even  crests  and  would  rise  to  a  common  level  but 
for  subsequent  warping  of  the  region.  At  any  stage  preced- 
ing extreme  old  age,  the  tops  of  individual  ridges  would  not 
have  been  even,  and  different  ridges  would  have  stood  at 
different  levels  (Why?).  Next,  the  plain  was  elevated  and 
warped  slightly  (A  — A  ),  without  further  folding  of  the  beds, 
thus  beginning  the  second  recorded  cycle  of  erosion.  The 
rejuvenated  streams,  together  with  the  new  tributaries  which 
worked  back  from  them,  now  opened  broad  valleys  on  the 
weaker  rocks  at  the  level  B — B,  above  which  the  stronger 
beds  stood  as  parallel  ridges.  Finally,  the  second  cycle  was 


FIG.  147.  —  The  even  sky  line  to  the  left  is  the  nearly  level  surface  of  a  pene- 
plain, which  bevels  across  sedimentary  and  igneous  rocks.  To  the  right 
it  cuts  tilted  Paleozoic  beds ;  to  the  left  pre-Cambrian  granite.  Since  the 
formation  of  the  peneplain  the  region  has  been  elevated  and  eroded. 
Western  Wyoming.  (Baker.) 

interrupted  and  the  third  one  begun  by  the  uplift  which  per- 
mitted the  streams  to  cut  the  new  valleys  at  C.  The  work 
of  reducing  the  area  to  base  level  in  the  present  cycle  remains 
largely  to  be  done.  The  few  new  valleys  of  the  larger 
streams  are  narrow  and  steep-sided.  The  region  is  accordingly 
in  the  youthful  stage  of  the  third  recorded  cycle  of  erosion. 
Figure  147  shows  a  nearly  horizontal  sky  line,  and  below 
it,  igneous  rocks  and  tilted  sedimentary  beds.  From  what 
has  preceded,  it  will  be  understood  readily  that  this  nearly 
level  surface  was  a  peneplain,  now  uplifted  and  dissected. 


THE  WORK  OP  STREAMS 


155 


Intrenched  meanders,  interrupted  profiles  of  streams, 
even-crested  mountain  ridges,  and  certain  kinds  of  terraces 
are  accordingly  among  the  features  which  may  indicate 
more  than  one  cycle  of  erosion,  and  which  often  aid  in  work- 
ing out  the  later  geological  history  of  a  region. 

Questions 

1.  At  what  stage  in  an  erosion  cycle  is  the  run-off  greatest  ? 

2.  At  what  stage  in  their   development  are  rivers  most  subject 
to  destructive  floods  ? 

3.  When  in  an  erosion  cycle  is  the  general  level  of  the  ground- 
water  surface  highest  ?    .  Lowest  ? 

4.  What  is  (1)  the  immediate,  and  (2)  the  final,  effect  of  stream 
erosion  upon  topography  ? 

5.  At  what  stage  of  an  erosion  cycle  is  agriculture  most  favored  ? 


FIG.  148.  —  View  in  the  western  part  of  the  province  of  Chi-li,  China.  The 
erosion  of  the  gneissic  rocks  has  been  aided  by  recent  deforestation. 
(Willis,  Carnegie  Institution.) 


156  PHYSICAL  GEOLOGY 

Least  favored?     At  what  stage  is  road  building  most  difficult? 
(Compare  Plates  III,  IV,  and  V.) 

6.  At  what  stage  in  the  life  of  a  river  is  it  most  likely  to  furnish 
water  power  ?     Navigation  ? 

7.  What  is  the  age,  in  terms  of  erosion,  of   the  area  shown  in 
Figure  148  ? 

8.  Make   a   diagram    showing    (1)    the    wet-weather   and    dry- 
weather  positions  of  the  ground-water  surface,  and   (2)  the  cross 
sections  of  valleys  which  (a)  frequently,  (6)  seldom,  and  (c)  never, 
"go  dry." 

9.  Compare   and   contrast    the   relief   of    two    topographically 
mature  regions  of  the  same  original  altitude,  one  of  which  is  near 
the  sea,  and  the  other  far  inland. 

10.  Assuming  that  the  rainfall  is  equal  everywhere  and  the  char- 

acter of  the  rocks  everywhere  the 
same,  what  change  will  occur  in  the 
position  of  the  divide  now  at  a  in  Fig- 
ure 149  ?  When  will  the  divide  cease 
to  shift  ?  How  will  the  character  of  the 

Fl°'  14°'  ^  dftST '  '    slopes  change  af ter  the  divide  becomes 

stationary  ?    What  would  be  the  effect 

if  the  rocks  to  the  left  of  a  were  harder  than  those  to  the  right  ? 

11.  What  would  be  the  width  of  valleys  if  they  were  due  only 
to  the  down  cutting  of  streams  ?     What  inferences  may  be  made 
from  the  fact  that  most  valleys  are  several  times  as  wide  as  they  are 
deep? 

12.  At  what  stage  in  its  life  should  a  river  be  engaged  chiefly  in 
deepening  its  valley  ?     In  widening  it  ? 

13.  What  is  the  age,  in  terms  of  erosion,  of  the  area  shown  in 
profile  in  Figure  150  ?     The  evidence  ? 


, S600' 


I          -  ^^ Y. I  5000' 

FIG.  150.  —  Profile  showing  the  character  of  the  surface  in  a  portion  of 
southeastern  Colorado.  Length  of  section  nearly  ten  miles.  Vertical 
scale  about  six  times  the  horizontal.  (U.S.  Geol.  Surv.) 


1600 

1200 

BOO 

400 

O 


FIG.  151.  —  Profile  of  an  area  in  West  Virginia,  near  Charleston.     Length 
of  section,  Q1^  miles.     Figures  show  elevation  above  sea  level. 

14.  In  what  stage  of  erosion  is  the  region  shown  in  profile  in 
Figure  151  ?  How  told  ?  What  is  the  age  of  the  valley  at  the  right 
end  of  the  section  ?  Of  the  rest  of  the  valleys  ? 


THE  WOKK  OF  STREAMS 


157 


15.    How  many  cycles  of  erosion  appear  to  be  shown  by  Figure  152  ? 
What  is  the  evidence  of  each  cycle  ?     Is  the  evidence  strengthened 


FIG.  152.  —  Profile  across  Dunning  and  Tussey  mountains,  Pennsylvania. 
Length  of  section  nearly  nine  miles.  Vertical  scale  about  two  and  one  half 
times  the  horizontal.  (U.S.  Geol.  Surv.) 

by  the  fact  that  the  beds  which  underlie  the  region  are  tilted? 
Reasons?     In  what  stage  of  the  present  cycle  is  the  region? 


FEATURES  DUE  TO  SPECIAL  STRUCTURES  AND  UNEQUAL 
HARDNESS 

Rapids  and  falls.  —  A  rapid  is  a  place  in  a  stream  where 
the  current  has  exceptional  velocity  (Fig.  153),  while  a  fall 
is  a  place  where  the  water  drops  (Figs.  154  and  155).  Rapids 


FIG.  153.  —  Chandlar  Rapids  in  Chandlar  River,  Alaska.      (Schrader, 
U.S.  Geol.  Surv.) 

and  falls  develop  in  various  ways,  the  more  important  of 
which  may  be  noted.  If  a  stream  formed  by  the  overflow 
of  a  lake  (p.  142)  were  to  flow  over  a  vertical  cliff,  a  fall 
would  result.  If  a  main  valley  were  deepened  by  a  glacier, 


FIG.   154.  —  Rainbow  Falls  of  Ausable  River,  New  York.    (U.S.  Geol.  Surv.) 
What  kind  of  rock  occurs  in  the  foreground  ?     What  appears  to  deter- 
mine the  position  of  the  little  cliff  which  extends  from  the  lower  left-hand 
corner  across  the  center  of  the  picture  ? 

(158) 


THE  WORK  OF  STREAMS  159 

while  its  tributary  valleys  were  not,  and  the  ice  were  to 
disappear  subsequently,  the  bottom  of  the  main  valley  would 
be  lower  than  the  mouths  of  the  tributaries,  whose  streams 
would  descend  by  rapids  or  falls.  Rapids  and  falls  of  this 
origin  are  common  in  some  of  the  mountains  of  western 
United  States  (p.  222).  Such  falls  are  consequent  upon  de- 


FIG.  155.  —  Twin  Falls  of  Snake  River,  Idaho.     (U.S.  GeoL  Surv.) 

clivities  which  the  rivers  had  no  share  in  forming,  and  so 
have  been  called  consequent  falls. 

A  second  class  of  falls  is  due  to  escarpments  which  the 
streams  helped  to  make ;  these  are  called  subsequent  falls. 
A  river  flowing  on  the  high  gradient  shown  in  Figure  156  is 
likely  to  be  a  degrading  river.  Obviously,  it  will  wear  its 
channel  faster  at  A,  where  the  rocks  are  soft,  than  just 
above,  where  they  are  hard.  The  result  is  that  the  gradient, 
and  hence  the  velocity,  of  the  stream  becomes  greater  at  A 
than  elsewhere  (Fig.  157).  In  other  words,  rapids  are  formed, 
which  become  increasingly  swift  as  the  gradient  becomes 


160 


PHYSICAL  GEOLOGY 


increasingly  steep.  Finally,  a  vertical  cliff  is  developed  at  A} 
over  which  the  stream  falls  (Fig.  158).  The  rapids  have 
been  replaced  by  a  waterfall.  The  falling  water  wears  the 
soft  rock  faster  than  the  hard  rock  which  overlies  it,  so  that 
projecting  ledges  of  the  hard  rock  are  formed.  From  time 


FIGS.  156,   157,  158.  —  Diagrams  to  illustrate  the  development  and  extinc- 
tion of  a  waterfall. 

to  time  pieces  fall  down  from  these  unsupported  ledges,  and 
by  this  process  of  undercutting  (sapping)  the  waterfall 
retreats  upstream.  Downstream  from  the  waterfall  the 
river  develops  a  slope  upon  which  it  is  at  grade.  It  is  evi- 
dent from  Figure  158  that  as  the  waterfall  retreats  upstream 
the  bottom  of  the  hard  fall-making  layer  approaches  the 
level  of  the  graded  channel.  Finally,  the  two  meet  (at  J5), 
and  the  waterfall  ceases  to  retreat  because  further  under- 


THE  WORK  OF  STREAMS 


161 


cutting  is  impossible.  The  point  of  maximum  wear  is  now 
transferred  to  the  brink  of  the  waterfall  at  C.  Wear  at  this 
point  presently  destroys  the  verticality  of  the  slope,  leaving 
a  steep  descent  down  which  the  water  flows  instead  of  fall- 


FIG.  159.  —  Diagram  showing  falls 
with  beds  dipping  upstream. 


FIG.  160.  —  Diagram  of  a  water- 
fall developed  on  vertical  beds. 


ing.     The  waterfall  has  been  succeeded  by  rapids.     Finally, 

the  top  of  the  hard  layer  is  eroded  to  the  level  of  the  graded 

channel,  and  the  rapids  disappear  (at  D,  Fig.  158).     (What 

would  be  the  effect  of  an  elevation 

of  the  valley  ?)    The  same  sequence 

of  events  would  occur  if  the  beds 

dipped  upstream    (Fig.   159),  but 

the    waterfall     would     retreat     a 

shorter  distance  and  would,  other 

things    equal,    be    shorter    lived. 

Waterfalls  may  develop  also  where 

the  beds  are  vertical   (Fig.  160). 

(How  would  the  history   of   such 

waterfalls  differ  from  the  history 

of  those    noted   before?)     Again, 

waterfalls  may  develop   on   beds 

dipping  gently,   but   not   steeply, 

downstream.      (May  there  be  rapids  where  the  beds   dip 

steeply  downstream?) 

Where  there  are  eddies  in  streams,  and  they  are  particu- 
larly common  at    the  bases  of    waterfalls  and  in   rapids, 
stones  may  be  whirled  round  and  round,  wearing  in  the  bed 
cylindrical,  well-like  depressions,  called  potholes  (Fig.  161) . 
B.  &  B.  GEOL.  — 10 


FIG.  161.  —  Pothole  in  bed  of 
stream  in  Smoky  Mountains, 
near  Hot  Springs,  North 
Carolina.  (Trowbridge.) 


162 


PHYSICAL  GEOLOGY 


Narrows.  —  When  the  rocks  in  the  sides  of  a  valley  arc 
of  unequal  strength,  the  valley  is  widened  at  unequal  rates 
at  different  points,  and  if  the  difference  in  the  character  of 

the  rocks  is  great,  the  valley 
may  become  wide  where  they 
are  weak,  while  still  narrow 
where  they  are  strong.  The 
places  where  valleys  have 
much  less  than  their  usual 
width  are  called  narrows  or 
water  gaps  (Figs.  162  and  163). 
Delaware  Water  Gap  and 
Harper's  Ferry  on  the  Potomac  are  among  the  more  famous 
of  many  narrows  in  the  Appalachian  Mountain  region.  Nar- 
rows develop  best  where  there  are  great  differences  in  the 
strength  of  the  rocks  which  form  the  valley  sides,  within 
short  distances.  They  are,  therefore,  usually  associated  with 
highly  tilted  beds  rather  than  with  horizontal  ones.  They 
are  most  conspicuous,  also,  in  connection  with  mature  valleys, 
for  very  young  valleys  are  narrow  everywhere,  and  very  old 
valleys  have  become  wide  everywhere,  regardless  of  the  char- 
acter of  the  rocks. 


FIG.  162.  —  Diagram  showing  water 
gaps. 


FIG.  163.  —  A  typical  water  gap  in  the  Appalachian  Mountains.  The 
Narrows  of  Wills  Mountain  at  Cumberland,  Md.  Several  roads  famous 
in  American  history  sought  the  West  through  this  gap.  (Aid.  GeoL  Surv.) 

Canons.  —  Valleys  that  are  strikingly  deep  in  relation  to 
their  width  are  called  gorges  (e.g.  Niagara  Gorge),  dells  (e.g. 
the  Dells  of  the  Wisconsin),  and  especially  in  the  West, 


jj^mH 


o. 


PLATE  VII.    PORTION  OF  THE  GRAND  CANON  OF  THE  COLORADO,  AND 

THE    MOUTH   OF  THE    CANON   OF  THE    LlTTLE    COLORADO    RlVER.      Contour 

interval,  50  feet.     Scale,  about  |  mile  per  inch.     (Vishnu.  Arizona,  Sheet, 
U-  $.  Geological  Survey.) 


THE  WORK  OF  STREAMS  105 

canons.  The  canon  of  the  Colorado  River  (Figs.  164  and 
165)  is  the  largest  in  the  world.  The  Colorado  has  been 
able  to  cut  a  very  deep  valley  because  the  surface  of  the 
plateau  in  which  it  is  formed  is  high  above  base  level.  The 
valley  is  still  narrow  because  (1)  the  climate  is  arid  arid  a 
number  of  the  agents  which  widen  valleys  (p.  140)  have 
accordingly  worked  slowly,  (2)  most  of  the  rocks  of  the 


FIG.  165.  —  Portion  of  the  Grand  Canon  of  the  Colorado   River,  Arizona. 
(Walcott,  U.S.  Geol.  Sun.} 

canon  walls  are  capable  of  standing  in  steep  faces,  and 
(3)  under  these  circumstances,  the  valley  has  not  existed 
long  enough  to  have  been  made  wide.  (How  can  the  river 
be  of  large  volume  when  the  climate  of  the  region  is  arid?) 
Vast  as  the  Colorado  Canon  and  its  tributary  canons  are 
(Plate  VII),  the  region  is  nevertheless  in  a  youthful  stage  of 
erosion,  for  very  little  of  the  work  of  reducing  it  to  base  level 
has  been  accomplished.  As  ages  pass,  the  canon  will  be 
worn  slowly  deeper  and  wider  by  the  water  and  weather, 
and  the  side  canons  will  become  larger  and  more  numer- 
ous, until  finally  the  great  plateau  will  be  reduced  to  a 
nearly  level  plain  but  little  above  sea  level.  (What  will  be 


166 


PHYSICAL  GEOLOGY 


FIG.  166.  —  A  canon  in  a  humid  region.     Val- 
ley of  the  New  River  in  the  Allegheny  Pla- 
teau. 
What  shows  that  the  climate  is  moist  ? 


the  topography  of  the  region  midway  between  the  present 
and  the  final  stage?)     The  conditions  which  brought  about 

the  formation  of  the 
Colorado  Canon  are 
those  which  most  fa- 
vor canon  develop- 
ment. They  are  (1) 
a  considerable  alti- 
tude, (2)  a  dry  cli- 
mate, (3)  rocks  that 
will  stand  in  cliffs, 
and  (4)  a  vigorous 
stream.  As  implied 
above,  however,  not 
all  canons  and  caiion- 
likq  valleys  are  in 
arid  regions.  Figure  166  shows  a  canon  in  a  moist  region. 
Rock  terraces.  —  Rock  terraces  (Fig.  167)  occur  on  the 
sides  of  many  valleys  cut  in  horizontal  beds  of  unequal 
strength.  The  terraces  are  formed  by  the  strong  beds, 
which  are  worn  back  less  rapidly  than  the  weak  beds  above 
and  below  them. 

Elevations  due  to  unequal  erosion.  —  The  relatively  rapid 
erosion  of  soft  rocks  has  left  associated  harder  rocks  stand- 
ing as  conspicuous  eleva- 
tions in  many  places.  If 
the  beds  are  tilted  highly, 
the  resistant  ones  may  be 
left  standing  as  ridges 
after  the  softer  ones  are  FlG'  167'  T  Diagram  °f  rock  terraces" 
worn  down  to  valleys  (Plate  XVI).  This  is  the  origin  of  the 
well-defined  Appalachian  Mountain  ridges  (p.  153).  Smaller 
ridges,  formed  in  this  way  on  the  flanks  of  mountain  ranges, 
are  sometimes  called  hogbacks  (Fig.  168).  The  superior  re- 
sistance of  dike  rock  may  lead  to  the  formation  of  dike  ridges 
(p.  50).  If  the  beds  are  horizontal,  flat-topped  table  moun- 


THE  WORK  OF  STREAMS 


167 


FIG.  168.  —  A  hogback.  East  flank  of 
Bighorn  Mountains,  Wyoming.  (Trow- 
bridge.) 


tains  may  develop.  In  the  western  part  of  the  United  States 
such  an  elevation  of  moderate  height  and  extent  is  often  called 
a  mesa  (a  Spanish  word 
meaning  table,  pronounced 
"  may-sa  "  ;  Figs.  169  and 
170).  Smaller  mesas  whose 
flat  tops  .  have  been  de- 
stroyed more  or  less  com- 
pletely by  erosion  are  fre- 
quently called  buttes  (a 
French  word  meaning  hill, 
pronounced  "bewts"; 
Fig.  171).  The  name  butte 
is  sometimes  applied 
loosely  in  the  West  to  any 
conspicuous  hill. 

Rock    structure    and 
stream  courses. — Joint 
systems  and  fissures  have  in  certain  places  guided  the  run- 
off    (Fig.     172),     producing 
drainage  systems  of  peculiar 
and  angular  pattern.     Many 
streams   in    the   Adirondack 
jVIountains  flow  along  inter- 
secting fault  lines.     In  a  re- 
gion underlain  by  horizontal 
m—^—-m        ~' 

FIG.   169.  —  Mesas.     Eastern  Ari- 
zona.    (Fairbanks.) 

beds,  lengthening  valleys  extend 
themselves  in  various  directions, 
and  the  stream  courses  are  with- 
out systematic  arrangement 
(Plate  IV).  In  an  area  of  tilted 
beds  of  unequal  strength,  many 
of  the  larger  streams  follow  the 


FIG.  170.  —  Mesa  Pino,  New 
Mexico. 


168 


PHYSICAL  GEOLOGY 


outcrops  of  the  weaker  layers,  while  their  tributaries  join  them 
at  right  angles,  producing  a  regular  drainage  pattern  (Fig.  173). 


FIG.  171.  —  Pawnee  Buttes,  Weld  Co.,  Colorado.      The  dark  beds  are  sand- 
stone ;   the  light  ones,  shale.     (Darton,  U.S.  Geol.  Sura.) 

In  such  a  region  streams  are  in  some  cases  diverted   from 
courses  across  hard  layers  to  courses  over  soft  layers.     The 

method  by  which  the 
change  is  accomplished 
may  be  illustrated  from 
Figures  174,  175,  and  176. 
In  Figure  174  the  farther 
stream  crosses  the  resistant 
ridge-making  layers  in 
water  gaps,  and  is  unable 
to  cut  its  valley  in  the 
weak  rock  just  above  the 
gaps  any  faster  than  it 
doe*sl^ii  the  hard  rock  at 
the  gaps.  The  nearer 
stream  does  not  cross  the 
hard  beds  and,  therefore, 
has  cut  its  valley  consider- 
ably lower,  and  is  lengthen- 
ing it  rapidly  by  headward 
erosion.  Presently  it  will 
reach  and  enter  the  farther 

FIG.  172.--  A   valley   formed    along  a 
joint    plane.       Enfield    Gorge,     near 

Ithaca,  N.Y.    (Tarr.) 


R  d   th  t  f 

J  ' 

the  latter  above  the  point 


THE  WORK  OF  STREAMS 


169 


of  invasion  will 
become  tribu- 
tary to  it,  for 
it  will  afford 
them  a  lower 
line  of  descent. 
In  Figure  175 
this  has  oc- 
curred. The 
process  by 
which  it  was 
accomplished 


FIGS.  174,  175,  176.  -  niaRrams  to  illus- 
trate  stream  piracy. 


FIG.  173.  —  Drainage  in  a  re- 
gion of  folded  rocks. 

is  known  as  stream  piracy. 
The  stream  which  effected 
the  capture  is  the  pirate, 
while  the  stream  which 
suffered  the  loss  is  called 
a  beheaded  stream.  The 
drainage  captured  is  said 
to  be  diverted.  The  sud- 
den increase  in  its  vol- 
ume has  permitted  the 
pirate  (Fig.  175)  to  cut 
a  new  valley  in  the  bot- 
tom of  its  old  one,  and 
the  remnants  of  the  old 
valley  flat  now  form  ter- 
races. Favored  by  the 
lower  level  afforded  by 
the  pirate,  the  diverted 
streams  have  lowered 

cept    above    the    upper 


170  PHYSICAL  GEOLOGY 

water  gap,  whose  hard  rocks  continue  to  act  as  a  base  level 
for  the  drainage  crossing  them.  It  is  evident  that  the  divide 
between  the  beheaded  stream  and  the  pirate  system  will  shift 
at  the,  expense  of  the  former,  for  the  slope  in  that  direction  is 
very  gentle,  while  that  toward  the  latter  is  steep.  The  result 
is  shown  in  Figure  176,  where  the  divide  has  migrated  to  a  per- 
manent position  at  the  outcrop  of  the  hard  rock.  The  be- 
headed river  now  rises  to  the  left  of  the  mountain  ridge, 
and  the  gap,  abandoned  by  the  stream  which  cut  it,  has 
become  a  wind  gap.  Wind  gaps  are  common  in  the  Appa- 
lachian Mountains.  Through  piracy,  streams  tend  so  far 
as  possible  to  get  off  the  hard  rocks  and  upon  the  soft 
rocks.  Thus  they  adjust  their  courses  to  the  structure  of 
the  beds.  The  result  is  appropriately  called  structural 
adjustment. 

While  stream  capture  is  most  common  in  regions  of  tilted 
or  folded  strata,  it  is  not  confined  to  them.  A  river  may 
be  able  to  capture  the  waters  of  neighboring  streams  because 
favored  by  larger  volume,  less  resistant  rock,  the  character 
or  amount  of  its  load,  or  a  shorter  course  to  the  sea. 

Questions 

1.  Why  are  steep  slopes  characteristic  of  arid  regions? 

2.  State  the  conditions  necessary  for  the  development  of  a  sub- 
sequent falls. 

3.  Enumerate  all  the  factors  upon  which  the  length  of  life  of  a 
given  waterfall  will  depend. 

4.  What  inference  concerning  the  structure  of  the  beds  under- 
lying a  given  region  may  be  made  :  (1)  From  the  fact  that  its  eleva- 
tions are  ridges  in  parallel  arrangement  ?     (2)  From  the  fact  that 
the    eastern    slopes    of    its  north-south  ridges   are  relatively  long 
and  gentle,  while  the  west-facing    slopes    are    short    and    steep? 
(3)  From  the  fact  that  its  elevations  are  without  systematic  ar- 
rangement ? 

5.  How  many  cycles  of  erosion  are  recorded  by  Figure  162  ? 

6.  (1)  What  is  the  age  of  the  valley  shown  in  Figure  177,  and  how 
is  it  shown  ?     (2)  What  kinds  of  work  is  the  river  doing  ?     (3)  How 
will  the  river  modify  its  valley  in  the  future?     (4)  What  was  the 
origin  of  the  waterfall  ?     (5)  What  will  be  its  future  ?     (6)  What 


THE  WORK  OF  STREAMS 


171 


evidence  is  there  of  weathering?  (7)  What  are  probably  the  chief 
agents  of  weathering  here  ?  (8)  Is  there  evidence  of  diastrophism  ? 
If  so,  what  ? 


FIG.  177.  —  South  Fork  of  Birch  Creek,  a  tributary  of  the  Yukon  River, 
Alaska.     (Prindle,  U.S.  GeoL  Surv.) 

STREAM  DEPOSITS 

Causes  of  deposition.  —  (1)  Anything  which  checks  the 
velocity  of  a  loaded  stream  occasions  the  deposition  of  sedi- 
ment,    (a)   A  decreasing  gradient  is  an  important   cause, 
especially  in  the  mid- 
dle   and    lower    por- 
tions of  large  valleys. 
(6)  Rivers  which  flow 
through     regions     of 
scant      rainfall      fre- 
quently    lose     water 
both  by  rapid  evapo- 
ration and  by  sinking 
into  the  ground  (Fig. 

178).        (Where   in   the     FlG>    178-  —  Tejunga    River,    southern    Cali- 
fornia,  sinking   in    the    sand    of   its    flood- 
United    States    IS  this         plain.     (Fairbanks.) 


172 


PHYSICAL  GEOLOGY 


the  case  over  large  areas?)  Diminished  volume  means 
reduced  velocity  and  carrying  power,  and  hence  deposition. 

(c)  Many  rivers   deposit 
at    their    mouths   where 
the   current  is   checked. 

(d)  Deposition  is  brought 
about  also  by  changes  in 
the  shapes  of  river  chan- 
nels.    If,    for    example, 
water  charged  with  sedi- 
ment  leaves    a    narrow, 
straight,  and  smooth  sec- 
tion  of   the   channel    to 
enter  a  wide,  crooked,  and 
irregular  one,  the  friction 
of  the  current  with  the 

bed  and  banks  is  increased,  its  velocity  is  therefore  decreased, 
and  deposition  may  result.  (2)  Tributaries  with  high  gradi- 
ents often  deliver  to  their  sluggish  main  streams  more  sedi- 
ment than  the  latter  can  wash  forward,  resulting  in  deposits 
along  the  floor  of  the  main  valley.  In  many  large  depositing 


FIG.  179.  —  An  alluvial  fan  in  the  Illinois 
Valley.  The  velocity  of  temporary, 
wet-weather  streams  is  reduced  as  they 
leave  the  gulley  in  the  background, 
and  they  are  forced'  to  deposit  the 
sediment  which  they  carry.  (III.  Geol. 
Surv.) 


FIG.  180.  —  Alluvial  fan  at  mouth  of  Aztec  Gulch,  Dolores  Valley,  south- 
western Colorado.     (U.S.  Geol.  Surv.) 
Account  for  the  small  fan  in  front  of  the  large  one. 


THE  WORK  OF  STREAMS  173 

rivers,  like  the  lower  Mississippi,  all  the  above  causes,  and 
perhaps  other  less  important  ones,  are  in  operation. 

The  principal  features  produced  by  stream  deposition  are 
described  in  the  following  paragraphs. 

Fans  and  cones.  —  Alluvial  fans  are  so  called  because 
they  are  half-circular  in  ground  plan  when  developed  typi- 
cally, and  are  composed  of  alluvial  material  (Figs.  179  and 
180).  Cones  are  relatively  steep  fans.  Alluvial  fans  vary 
in  diameter  from  a  few  feet  to  several  miles.  Some  of  the 
California  rivers  have  built  fans  some  forty  miles  across. 
Fans  are  developed  best  at  the  bases  of  steep  slopes  in  dry 
regions,  where  streams  of  diminishing  volume  leave  the  rela- 
tively high  gradients  of  their  mountain  valleys  to  enter  low- 
lands. The  deposit  in  such  a  situation  chokes  the  channel 
of  the  stream,  and  some  of  the  water  spreads  around  the 
obstruction.  The  process  being  repeated  many  times,  and 
the  stream  meanwhile  extending  the  deposits  in  the  direction 
of  its  flow,  they  presently  acquire  more  or  less  of  the  "  fan  " 
shape  which  suggested  their  name.  The  main  water  chan- 
nels of  many  large  fans  give  off  branches  that  in  turn  di- 
vide repeatedly  downstream.  These  branching  channels  are 
called  distributaries,  and  their  explanation  is  involved  in 
what  has  already  been  said.  The  deposits  in  a  given  chan- 
nel reduce  its  size  until  some  of  the  water  breaks  over  the 
side  and  follows  a  new  course  to  the  margin  of  the  fan. 
The  new  channel,  becoming  choked,  gives  off  other  distribu- 
taries, which  divide  again.  The  spreading  of  the  water 
flowing  over  the  fan  becomes  an  important  cause  of  deposi- 
tion, since  it  increases  the  friction  of  flow,  and  therefore 
decreases  the  velocity.  Deposition  may  be  caused  also  by 
much  or  all  of  the  water  sinking  into  the  porous  material 
of  the  fan.  Thus  the  growth  of  fans  is  due  to  deposition 
brought  about  by  (1)  decrease  in  gradient,  (2)  increase  in 
friction  of  flow,  and  (3)  often  by  decrease  in  volume.  Plate 
VIII  shows  a  portion  of  a  large  fan,  together  with  waste 
channels,  tributaries,  and  distributaries. 


PLATE  VIII.  PORTION  OF  A  LARGE  ALLUVIAL  FAN  IN  SOUTHERN  CALI- 
FORNIA. Contour  interval,  50  feet.  Scale,  about  1  mile  per  inch.  (San 
Bernardino,  California,  Sheet,  U.  S.  Geological  Survey.) 


THE  WORK  OF  STREAMS  175 

The  structure  of  alluvial  fans  is  characteristic,  and  results 
from  the  method  of  their  growth.  The  coarsest  material  is 
dropped  at  the  apex  of  the  fan,  where  the  current  is  first 
checked,  and  the  deposit  made  at  any  given  time  becomes 
progressively  finer  toward  the  margin.  This  does  not  mean 
that  the  material  in  a  vertical  section  through  an  alluvial 
fan,  all  parts  of  which  are  at  the  same  distance  from  the 


FIG.  181.  —  Section  of  an  alluvial  fan,  Owens  Valley,  Cal.     (Trowbridge.) 

apex,  is  all  of  the  same  degree  of  coarseness.  On  the  con- 
trary, the  material  would  probably  change  frequently, 
both  horizontally  and  vertically,  for  the  volume  (and  so  the 
carrying  power)  of  different  distributaries  would  vary  at 
the  same  time,  and  that  of  any  given  distributary  at  differ- 
ent times.  Such  variations  in  the  tops  of  fans  may  often 
be  seen  in  the  sides  of  the  channels  which  trench  them  (Fig. 
181). 

The  angle  of  slope  of  a  fan  depends  upon  how  suddenly 
and  how  much  the  velocity  of  the  depositing  waters  was 
diminished,  and  upon  the  kind  and  amount  of  material 
they  carried.  A  sudden  and  great  reduction  in  the  velocity 
of  a  stream  heavily  loaded  with  coarse  material,  gives  a 
relatively  steep  slope;  the  opposite  combination  a  gentle 
one.  The  profile  of  a  fan  along  any  radius,  like  the  profiles 
of  other  depositional  slopes,  is  a  curve  concave  upwards 


176 


PHYSICAL  GEOLOGY 


(Fig.  182).  (What  would  be  the  character  of  a  curve  drawn 
on  the  surface  of  a  fan  along  a  line  all  points  in  which  were 
equidistant  from  the  apex  of  the  fan?) 


FIG.  182. 


Profile  of  a  large  alluvial  fan  near  Cucamonga,  Cal. 
section,  6V2  miles. 


Length  of 


The  growing  fans  of  neighboring  streams  in  arid  regions 
unite  in  many  cases  to  form  extensive  alluvial  slopes  or 
plains  (Fig.  183). 

Certain  rivers  have  been  ponded  back  by  the  fans  of 
tributaries,  forming  broad,  lakelike  expansions  of  the  river. 


FIG.  183.  —  A  piedmont  alluvial  plain,  Silver  Peak  Range,  Nevada.  Waste 
from  the  mountain  valleys  unites  to  form  a  compound  fan.  (Sketch  from 
photograph  by  Spurr,  U.S.  Geol.  Sitrv.) 

Lake  Pepin  in  the  upper  Mississippi,  and  Lake  Peoria  in  the 
Illinois  River  (Plate  II),  are  of  this  origin. 

Flood  plains.  —  The  portion  of  a  valley  bottom  subject 
to  inundation  is  called  the  flood  plain  (Plate  IX).  Flood 
plains,  or  flats,  are  usually  formed  primarily  by  the  side 
cutting  of  relatively  sluggish  streams  (p.  140),  and  subordi- 
nately  by  deposits  made  during  overflow.  In  exceptional 
cases  rivers  occupying  narrow-bottomed  valleys  are  forced 
to  aggrade  their  channels,  and  flood  plains  rasult  that  are 
due  entirely  to  deposition  (Fig.  184).  The  alluvial  deposit 
may  cover  the  underlying  rocks  thinly  or  thickly. 


THE  WORK  OF  STREAMS 


177 


Normal  flood  plains  are  widest  in  their  lower  portion, 
where  the  gentle  gradient  favorable  to  lateral  shifting  was 
developed  first,  and  be- 
come narrower  more  or 
less  regularly  up  valley. 
The  lower  Mississippi  has 
opened  a  flood  plain  from 
20  to  60  miles  or  more 
wide.  The  downstream 
slope  of  flood  plains  varies  FIG. 
with  the  volume  of  the 
stream  and  the  character 
of  the  material  it  deposits. 
Relatively  small  streams 
heavily  overloaded  with 


184.  —  Diagram    of    a    flood     plain 
formed  by  deposition  in  a  narrow  valley. 

What  is  the  age  of  the  rock  valley  in 
which  the  filling  has  occurred,  and  how  is 
it  shown  ?  What  work  was  the  river  do- 
ing before  filling  commenced  ?  The  evi- 
dence? What  things  may  have  forced 
the  Driver  to  cease  its  earlier  work  and  ag- 
grade its  valley  ? 


coarse  material  build 
steep  flood  plains,  sometimes  with  a  descent  of  5Q:  to  75 
feet  a  mile;  large  rivers,  depositing  fine  sediment,  build 
nearly  level  flats. 

Natural  levees.  —  During  times  of  flood  a  river  deposits 
most  actively  along  the  edges  of  the  channel.  Here  the 
depth  of  the  overflowing  water  is  diminished  suddenly  and, 
in  consequence,  its  velocity  and  carrying  power.  Here 
during  the  continuance  of  the  overflow  the  marginal  waters 
of  the  main  current  are  checked  by  friction  with  the  slower 
moving  backwaters.  Deposition  along  these  lines  during 
many  overflows  may  build  low,  marginal  ridges  with  a 
gentle  slope  away  from  the  river.  Such  embankments  are 

natural  levees  (Fig. 
185).  It  is  evident 
that  natural  levees 
will  not  prevent 
subsequent  over- 
flow, since  a  stream 


, 

.   185.  —  Diagram  showing  natural  levees  and 


can  build  them  only 


.  . 

the  general  structure  of  stream-laid  beds.  to   the   level    of    its 


178  PHYSICAL  GEOLOGY 

flood  waters.  Artificial  embankments  have  been  built  upon 
the  natural  levees  of  many  rivers,  in  order  to  reclaim  their 
bottom  lands.  As  deposition  continues  along  the  bed  of  the 
river,  such  embankments  must  be  built  higher  from  time  to 
time  in  order  to  confine  the  stream. 

The  low  margins  of  many  wide  flood  plains  are  marshy. 
In  such  marshes,  the  dead  leaves,  twigs,  and  branches  of 
the  swamp  vegetation  gather  in  the  shallow  water,  along 


FIG.  186.  —  Meanders  of  the  upper  Green  River,  Wyoming.     (Baker.) 

with  minor  quantities  of  silt.  This  vegetal  matter,  preserved 
by  the  water  from  complete  decay,  may  be  transformed 
slowly  into  peat.  Some  coal  beds  are  thought  to  represent 
similar  marshes  which  existed  ages  ago  (pp.  378,  380). 

Many  tributary  streams  on  entering  an  aggraded  valley 
are  prevented  by  the  natural  levees  from  uniting  with  the 
main  river  at  once,  and  flow  greater  or  lesser  distances  down 
valley  before  joining  it  at  some  point  where  it  swings  to  their 
side  of  the  flood  plain  (Plate  IX). 

Braided  rivers.  —  In  some  cases  the  waters  of  rapidly 
depositing  rivers  flow  in  numerous  channels  which  meet  and 
divide  repeatedly.  Deposition  along  the  floor  of  a  given 
channel  reduces  its  capacity.  When  the  channel  is  presently 
unable  to  hold  all  of  its  water,  a  part  breaks  over  the  side 
and  follows  a  new  line.  The  new  channel,  becoming  choked 
like  the  old  one,  gives  off  branches  which  in  turn  divide. 
The  overflowing  waters  follow  the  lowest  accessible  lines  of 


THE  WORK  OF  STREAMS  179 

descent,  and  may  reunite  only  to  separate  again  a  little 
farther  down  valley.  By  this  process  the  river  is  split  into 
many  minor  streams  which  shift  continually  and  inclose 
changing  islands  of  sand  and  gravel.  Such  rivers  are  braided 
rivers. 

Stream  meanders  and  flood-plain  lakes.  —  Even  if  nearly 
straight  in  the  beginning,  a  river  must  come  to  follow  a 


FIG.  187.  —  Meanders  of  the  Jhelum  River  in  the  valley  of  Kashmir,  India. 

serpentine  (meandering)  course  (Figs.  186  and  187)  on  a 
flood  plain  of  low  slope.  This  results  primarily  from  the 
fact  that  its  sluggish  current  is  turned  against  the  banks 
easily  by  irregularities  of  the  channel,  by  the  currents  of 
tributary  streams,  and  in  other  ways.  The  current  cuts 
into  the  banks  where  it  strikes  them.  As  it  issues  from  a 
cut  in  the  bank,  it  is  directed  against  the  opposite  bank  a 
little  farther  downstream,  and  forms  a  curve  there.  The 
development  of  this  bend  leads  to  the  formation  of  another, 
and  so  on.  As  erosion  continues,  the  cuts  tend  to  become 
smooth  curves,  better  adapted  to  the  regular  movement  of 
the  current.  At  the  same  time  the  stream  erodes  these 
curves,  where  the  current  is  relatively  swift,  it  builds  up  to 
flood  level  the  opposite  side  of  the  channel,  where  the  water 
is  slack.  In  this  way  it  comes  to  follow  a  more  or  less  regu- 
larly curved  course  suited  to  its  volume  and  gradient.  As 


180 


PHYSICAL  GEOLOGY 


the  process  of  cut-and-fill  continues,  the  curves  change  in 
outline  as  suggested  by  Figure  188.  Finally  the  stream  cuts 
through  the  narrowing  neck  of  land  between  the  two  limbs 

of  a  meander  (Fig.  189).  The  cur- 
rent now  abandons  the  old  round- 
about course  because  the  new 
route  is  steeper.  The  old  channel 
is  isolated  presently  by  the  shift- 
ing of  the  stream  to  another  posi- 
tion on  its  flood  plain,  or  by 
at  the  ends  of  the 


FIG.    188.  —  Diagram    showing 
development   of    a    meander. 
The  current  directed  against  deposition 
the  downstream   side  of    the  .         ,          ,. 
meander    is    on    the   average  abandoned  meander,  Whose  Stand- 
stronger  than  that  directed  ing 'waters  check  the  edge  of  the 

against    the    upstream    side,  m,  ,,.         ,    , 

and   therefore   the   growing  current.     The  resulting  lake  is  an 

meander  migrates   down  the    ox-bow  lake.      The  flood  plain   of   a 

great  river,  such  as  the  Mississippi 

or  Missouri  (Plate  IX),  may  contain  numerous  lakes,  which 
record  recent  changes  in  the  position  of  the  river  (Fig.  141). 
The  extent  to  which  certain  great  rivers  are  shifting  thei;- 
channels  is  shown  by  surveys  of  their 
courses.  Figure  190  shows  the  changes 
that  occurred  in  the  position  of  a  por- 
tion of  the  Missouri  River  between 
1852  and  1879,  and  between  the  latter 
date  and  1894.  It  shows  also  the 
tendency  of  the  meanders  to  work 
down  the  valley. 

Ox-bow  lakes,  like  lakes  of  other 

-  rrn.          FIG.   189.  —  A  recently  de- 

origin,  are  temporary  features.    They  veloped  cut-off, 

are  filled  gradually  (1)  by  the  en-  What  shows  that  it  is  of  re- 
croachment  of  marsh  vegetation  upon 

their  shallow  borders,  (2)  by  silts  deposited  in  them  during  ex- 
ceptional floods,  (3)  by  wind-blown  material,  and  (4)  by  wash 
from  the  surrounding  land.  Doubtless  many  generations  of 
lakes  are  made  and  destroyed  during  the  formation  of  great 
flood  plains. 


182 


PHYSICAL  GEOLOGY 


The  materials  and  structures  of  flood  plains.  —  As  already 
implied,  the  materials  of  flood  plains  range  from  coarse  gravel 
to  finest  mud.  The  coarser  material  deposited  by  a  river  is 

confined  in  general  to  the 
vicinity  of  the  channel, 
where  the  velocity  of  the 
overflow  is  checked  first 
and  most.  This  grades 
more  or  less  irregularly 
into  the  fine  muds  which 
gather  in  the  quiet  back- 
waters. When  the  river 
changes  its  position  on 
its  valley  floor,  the  coarser 
deposits  along  the  new 
channel  cover  finer  de- 
posits made  at  a  distance 
from  the  old  channel, 
whose  coarser  material  is 
in  turn  buried  with  fine. 
Frequent  changes  in  the 
position  of  the  aggrading 


FIG.  190.  —  Map  showing  changes  in  the 
course  of  a  portion  of  the  Missouri 
River. 


river  result  in  many  ver- 
tical alternations  in 
coarseness  among  its  sed- 
iments. Minor  variations 
may  be  brought  about  by  the  unequal  strength  of  the 
overflow,  capable  of  moving  particles  of  varying  size  to  a 
given  place  at  different  times.  Further  complexity  in  the 
distribution  of  the  materials  of  a  flood  plain  is  introduced  by 
irregular  contributions  made  by  wash  frojft  the  bluffs  and 
by  tributary  streams.  Figure  185  shows  the  general  structure 
of  stream-laid  beds. 

The  structure  described  above  has  made  it  possible  to 
determine  that  certain  ancient  formations  were  laid  down 
on  the  land  by  rivers,  and  not  in  lakes  or  the  sea.. 


THE  WORK  OF  STREAMS 


183 


FIG.  191.  —  Diagram  of  high  alluvial 
terraces. 


Alluvial  terraces.  —  Under  new  conditions,  a  river  which 
has  been  depositing  may  find  itself  underloaded.  The 
change  may  be  due  to  a  movement  of  its  valley  resulting 
in  a  steeper  gradient,  to  an  increase  of  volume,  to  a  decrease 
in  the  amount  of  sediment 
received  from  its  head- 
waters, or  to  still  other 
causes.  Whatever  the 
cause,  the  river,  if  greatly 
underloaded,  sinks  its 
channel  rapidly.  The 
remnants  of  the  old  flat 
then  stand  as  alluvial  terraces  on  one  or  both  sides  of  the 
valley  (Fig.  191).  After  the  river  has  opened  out  a  new  flood 
plain  at  a  lower  level,  for  some  reason  it  may  again  degrade 
actively,  leaving  a  second  set  of  terraces.  Indeed,  this  pro- 
cess may  be  repeated  a  number  of  times.  If  a  river  which 
has  been  aggrading  is  able,  under  changed  conditions,  to  de- 
grade, but  remains  nearly  loaded,  it  may  shift  from  side  to 
side  of  its  valley  while  it  slowly  lowers  its  channel,  and  by 
this  means  form  a  series  of  terraces.  This  is  illustrated  in 
Figure  192,  where  a  stream  is  supposed  to  have  filled  its  valley 

to  the  level  A-D-B,  and 
to  occupy  a  position  near 
the  left  edge  of  its  flood 
plain,  at  A.  If  the  stream 
now  shifts  toward  the  op- 

FIG.    192.  —  Diagram    to    illustrate    the  .,        >-,        f    ,,  n 

formation  of  terraces  by  a  river  which  is    POSlte   Slde    °f    the   valley, 
degrading  slowly,  and  shifting  from  side    meanwhile      degrading,      it 

will  occupy  presently  the 

position  C.  Should  movement  to  the  right  stop  there,  be- 
cause of  contact  with  a  projection  of  the  valley  wall,  or 
for  some  other  reason,  and  the  river  return  toward  the  left 
side  of  the  valley,  a  remnant  of  the  old  flood  plain,  C-D-B, 
would  remain  as  a  terrace.  In  similar  manner,  should 
the  river  fail  to  reach  the  left  side  of  its  valley  on  the  re- 


184 


PHYSICAL  GEOLOGY 


turn  swing,  a  terrace  would  result,  as  at  E-F-A.  Many 
terraces  at  successively  lower  levels  might  result  from  a 
continuation  of  this  process.  These  terraces  might  extend 

a  considerable  dis- 
tance along  the  val- 
ley, or  only  a  short 
distance,  and  their 
width  might  vary 
notably.  It  is  evi- 
dent that  when 
formed  in  this  way, 
terraces  upon  oppo- 
site sides  of  a  valley 
will  not  correspond  in 
elevation.  Small  ter- 
races  are  common 
even  in  young  val- 
leys, where  they  are 
due  in  many  cases  to 
the  fact  that,  as  the 
streams  degraded, 
they  also  shifted  their 
positions  laterally. 

Terraces  may  be  destroyed  wholly  or  in  part  by  the  widen- 
ing of  the  flood  plain  at  a  lower  level.     Indeed,  since  the 
goal  of  stream-borne  waste  is  the  sea,  the  depositional  fea- 
tures discussed  in  the  preceding 
paragraphs  may  all  be  regarded 
as  composed  of  material  which 
has  been  dropped  only  tempo- 
rarily  by   overloaded    streams, 
and  which  sooner  or  later  will 
resume  its  journey  to  the  ocean. 
Many  cities  are  located  partly 
or  wholly  upon  the  terraces  of 
great    rivers.     Peoria,     Illinois 


FIG.  193.  —  Delta  of  the  Mississippi  River. 


FIG.  194.  —  Delta  of  the  Nile 
River.  The  dotted  area  is 
desert. 


THE  WORK  OF  STREAMS 


185 


(Plate  II) ;  Dubuque,  Iowa ;  and  Hartford,  Connecticut,  are 
examples.  Miller  and  Crown  City  (Plate  IV)  are  examples 
of  hundreds  of  villages  situated  similarly. 

Deltas.  —  Some  of  the  material  which  rivers  bring  to  the 
sea  or  to  lakes  is  carried  away  by  waves  and  currents ;  much 
of  it  often  accumulates  off  the  mouths  of  the  rivers,  especially 
if  they  flow  into  tideless  or  nearly  tideless  bodies  of  water. 


FIG.  195.  —  The  delta  of  the  Alsek  River,  Alaska.     Shows  numerous  dis- 
tributaries.     (Netland,   U.S.  Boundary  Commission.) 

Such  deposits  may  form  deltas  (Fig.  93).  Deltas  are  so 
called  from  the  Greek  letter  (A)  of  that  name,  whose  shape 
they  occasionally  resemble  (Fig.  194). 

As  the  current  is  checked  at  the  mouth  of  a  river  flowing 
into  the  sea,  the  coarsest  of  the  sediment  is  dropped  first, 
forming  slanting  beds,  whose  angle  of  slope  is  determined 
largely  by  the  size  and  shape  of  the  material.  The  finer 
sediment  settles  less  rapidly,  and  is  spread  by  waves  and 
tides  over  a  larger  area  in  nearly  horizontal  sheets.  As  dep- 
osition continues,  the  steeper  beds  of  coarser  material  are 
built  out  upon  the  nearly  level  beds  of  fine.  When  this 
submarine  embankment  is  built  up  close  to  the  surface  of 
the  water,  it  becomes  in  effect  an  extension  of  the  river  bed, 
across  which  the  projected  current  rolls  and  drags  material, 
and  upon  which  it  deposits  a  part  of  its  load.  Deposition 
on  the  submarine  platform  is  most  active  along  the  edges 
of  the  river  current,  because  of  friction  with  the  relatively 
B.  &  B.  GEOL.  — 11 


186  PHYSICAL  GEOLOGY 

quiet  sea  water.  Thus  levees  develop,  and  the  delta  is 
built  above  the  level  of  the  sea.  As  the  original  channels 
across  the  delta  gradually  fill  with  sediment,  some  of  the 
water  breaks  over  the  sides,  following  new  courses  to  the  sea 
and  building  up  the  different  parts  of  the  delta  in  turn. 
By  this  means  a  complicated  system  of  distributaries  may 
be  formed  (Fig.  195).  Portions  of  the  shallow  sea  covering 
the  submarine  platform  are  sometimes  inclosed  by  the  river 
deposits  or  between  them  and  the  old  shore  line,  forming 
delta  lakes.  This  was  the  origin  of  Lakes  Borgne  and 
Pontchartrain,  on  the  delta  of  the  Mississippi  near  New 
Orleans  (Fig.  193).  Extensive  deposits  of  peat  may  accu- 
mulate in  delta  lakes  and  swamps.  Apart  from  the  shallow 
basins  of  their  lakes  and  marshes,  and  the  low  ridges  along 

their  distributaries,  the 
land  surfaces  of  great 
deltas  are  nearly  level, 

FIG.  196.  -  Profile  and  section  of  a  delta.    continuing  the  glope  of  the 

flood  plain  farther  up  river.  The  upper  beds  of  a  delta,  de- 
posited by  the  river  upon  the  submarine  flat,  are  nearly  hori- 
zontal. Deltas  are  accordingly  characterized  by  three  sets 
of  beds  (Fig.  196).  The  bottom  and  top  beds  are  nearly 
horizontal,  while  the  middle  beds  are  inclined  more  or  less 
steeply. 

Deltas  grow  at  very  unequal  rates.  The  ratio  between  the 
volume  of  sediment  brought  by  the  river  and  the  strength  of 
waves  and  currents  off  the  river  mouth  is  a  chief  determinant. 
The  Mississippi  brings  down  about  7,500,000,000  cubic  feet 
of  sediment  a  year ;  and  as  the  tides  of  the  Gulf  of  Mexico 
are  weak,  the  delta  is  being  extended  seaward  off  the  mouths 
of  the  main  distributaries  at  the  rate  of  about  a  mile  in  sixteen 
years.  It  appears  to  have  grown  at  about  this  rate  for  many 
years.  An  English  writer  reported  in  1770  that  the  Balize,  a 
small  fort  built  by  the  French  on  a  little  island  which  was  at 
the  mouth  of  the  river  in  1734,  was  then  two  miles  up.  The 
depth  of  the  water  into  which  a  delta  is  being  built  also  in- 


THE  WORK  OF  STREAMS  187 

fluences  the  rate  of  its  forward  growth.  Furthermore,  great 
deltas  are  as  a  rule  sinking  slowly,  and  the  relation  of  up- 
building to  subsidence  varies  greatly.  In  some  cases,  for 
example  the  Mississippi  and  Ganges,  rivers  have  built  up 
their  deltas  faster  than  the  region  has  subsided.  In  other 
cases,  subsidence  is  so  rapid  as  to  prevent  the  building  of  deltas 
above  the  sea.  In  the  Chesapeake  Bay  region  recent  subsi- 
dence has  formed  great  estuaries,  and  the  rivers  are  now 
building  marshy  bay-head  deltas.  The  delta  of  the  Mississippi 
has  an  area  of  over  12,000  square  miles,  and  the  compound 
delta  of  the  Ganges  and  Brahmaputra  rivers  is  between 
50,000  and  60,000  square  miles  in  extent  (about  as  large  as  the 
state  of  Illinois) .  As  a  result  of  long-continued  subsidence  and 
up-building,  delta  deposits  may  attain  great  thickness. 

Ancient  delta  beds  of  great  thickness,  their  origin  revealed 
by  their  structure,  occur  in  certain  localities,  —  for  example,  in 
the  vicinity  of  Puget  Sound.  They  afford  a  record  of  the 
physical  geography  of  the  region  at  the  time  when  the  sedi- 
ments, later  changed  into  firm  rocks,  were  deposited. 


1.  How  could  one  distinguish  in  the  field  between  an  ancient 
alluvial  fan  and  an  ancient  delta  ? 

2.  (1)  What  occasioned  the  building  of  the  fan  shown  in  Figure 
197  ?     (2)  Is  the  front  of  the  fan  the  same  as  when  built?     (3)  If 
not,  how  has  it  been  changed,  and  by  what  ?     (4)  Account  for  the 
trench  which  crosses  the  fan.     (5)  How  may  the  miniature  terraces 
within  the  trench  be  ex- 
plained ? 

3.  What  are  the  gen- 
eral conditions  which  oc- 
casion the  development 
of  distributaries  ? 

4.  What  are  all   the 
ways  in  which  Plate  IX 
shows  that  the  Missouri 

River  is  there  a  deposit-     pIG.  197.  _  A  small  fan  on  the  beach  of  Lake 
ing  stream?  Michigan. 


188 


PHYSICAL  GEOLOGY 


5.   Compare   the    downstream  slope  of    the  higher  and  lower 
terraces  of  a  given  valley. 

6.    Interpret  Figure  198,  indicating  (1)  the  successive  steps  in  the 
development  of  the  features  shown,  and  (2)  how  the  several  changes 


FIG.  198.  —  Diagram  of  stream  terraces. 

that  are  recorded  in  the  work  of  the  river  may  have  been  brought 
about. 

7.  Why  are  the  materials  brought   by  great  rivers   to  the  sea 
usually  fine  (though  unequally  so)  ? 

8.  Interpret  the  fact  that  limestones   containing  marine  fossils 
are  sometimes  found  interbedded  with  delta  deposits. 

SUMMARY 

The  mission  of  running  water  is  to  wear  the  land  to  base 
level.  The  material  it  carries  toward  and  to  the  sea  is  pre- 
pared for  transportation  largely  by  the  agents  of  weathering, 
and  in  subordinate  amount  is  worn  from  the  rocks  by  the 
streams  themselves.  The  irregular  reduction  of  the  land 
produces  most  of  the  familiar  relief  features  of  the  surface, 
whose  characteristics  are  determined  by  several  factors, 
especially  by  the  character  and  structure  of  the  rocks  from 
which  they  were  carved,  and  the  stage  of  development  which 
they  have  reached.  The  waste  of  the  land  is  often  laid  aside 
on  its  way  to  the  sea  by  overloaded  streams,  forming  topo- 
graphic features  subject  to  later  destruction  by  eroding  waters 
or  by  other  agencies. 

The  getting  of  the  land  into  the  sea  has  been  the  great  task 
of  streams  throughout  all  the  geological  ages  since  lands  and 
seas  existed,  and  the  materials  of  the  sedimentary  rocks  of 
existing  lands  represent  for  the  most  part  the  stream-borne 


THE  WORK  OF  STREAMS  189 

waste  of  ancient  lands,  brought  to  shallow  seas  which  occupied 
the  areas  where  the  rocks  occur.  Ancient  peneplains  and 
other  phenomena  show  that  at  various  times  in  different  places 
the  streams  of  past  ages  have  nearly  completed  their  task, 
only  to  have  it  renewed  when  their  basins  were  rejuvenated  by 
a  sinking  of  the  sea  or  by  an  uplift  of  the  land. 

It  is  evident  from  the  preceding  pages  that  the  activities 
of  streams  are  of  prime  importance  in  shaping  the  present 
chapter  in  the  history  of  the  earth.  It  will  be  seen  in  sub- 
sequent pages  that  the  results  of  stream  activity,  with  which 
the  student  is  now  familiar,  are  likewise  of  prime  importance 
in  deciphering  the  earlier  chapters  of  the  earth's  history. 

REFERENCES 

BONNET  :    Rain  and  Rivers  as  Sculptors  and  Rivers  as  Transporters, 

in  The  Story  of  our  Planet,  pp.  103-142.     (London,  1893.) 
DARTON  :    Examples  of  Stream-robbing  in  the  Catskill  Mountains,  in 

Bull.  Geol.  Soc.  of  Am.,  Vol.  VII,  pp.  505-507. 
DAVIS  :    The  Seine,  the  Meuse,  and  the  Moselle,  in  Nat.  Geog.  Mag., 

Vol.  VII,  pp.  189-202,  228-238. 
The  Rivers  and  Valleys  of  Pennsylvania,  in  Nat.  Geog.  Mag., 

Vol.  I,  pp.  183-253. 
Stream  Contest  along  the  Blue  Ridge,  in  Bull.  Geog.  Soc.  of 

Phil.,  Vol.  Ill,  pp.  213-244. 
Geographic  Cycle  in  an  Arid  Climate,  in  Jour,  of  Geol.,  Vol. 

XIII,  pp.  381-407. 
—  The   Development  of  River   Meanders,   in  Geol.   Mag.,  N.   S., 

Decade  IV,  Vol.  X,  pp.  145-148. 
-  River  Terraces  in  New  England,  in  Bull.  Harvard  Mus.  of  Comp. 

Zool.,  Vol.  XXXVIII,  pp.  281-346. 

—  The  Physical  Geography  of  Southern  New  England,  in  Physiog- 
raphy of  the  United  States,  pp.  269-304.     (New  York,  1895.) 
DAVIS  and  WOOD:    The  Geographic  Development  of  Northern    New 

Jersey,  in  Proc.  Bost.  Soc.  Nat.  Hist.,  Vol.  XXIV,  pp.  365-423. 
DODGE  :    The  Geographical  Development  of  Alluvial  River    Terraces, 

in  Proc.  Bost.  Soc.  Nat.  Hist.,  Vol.  XXVI,  pp.  257-273. 
DUTTON  :    Tertiary    History  of   the  Grand  Canon  District;     Mono. 

II,  U.S.  Geol.  Surv. 
GANNETT:    Profiles  of  Rivers  in  the  United  States;    Water   Supply 

and  Irrigation  Paper  No.  44,  U.S.  Geol.  Surv. 


190  PHYSICAL  GEOLOGY 

GILBERT  :    Lind  Sculpture,  in  Geology  of  the  Henry  Mountains. 

pp.  99-150 ;  U.S.  Geog.  and  Geol.  Surv.,  Rocky  Mtn.  Region. 

(Washington,  1877.) 
Niagara  Falls  and  their  History,  in  Physiography  of  the  United 

States,  pp.  203-236.     (New  York,  1895.) 
GOODE  :    The  Piracy  of  the  Yellowstone,  in  Jour,  of  Geol.,  Vol.  VII, 

pp.  261-271. 
HAYES  :   The  Southern  Appalachians,  in  Physiography  of  the  United 

States,  pp.  305-336.     (New  York,  1895.) 
—  Physiography  of  the  Chattanooga  District,  in  19th  Ann.  Rept., 

U.S.  Geol.  Surv.,  Pt.  II,  pp.  1-58. 
JEFFERSON  :    Limiting  Width  of  Meander  Belts,  in  Nat.  Geog.  Mag., 

Vol.  XIII,  pp.  373-384. 
JOHNSON,  L.  C.  :    The  Nita  Crevasse,  in  Bull.  Geol.  Soc.  of  Am., 

Vol.  II,  pp.  20-25. 

POWELL  :  Exploration  of  the  Colorado  River  of  the  West  and  its  Tribu- 
taries.    (Washington,  1875.) 

RUSSELL  :    Rivers  of  North  America.     (New  York,  1898.) 
SALISBURY  :    The   Physical   Geography    of  New  Jersey;    N.J.  Geol. 

Surv.,  Vol.  IV,  pp.  65-154. 
SALISBURY  and  ATWOOD  :    The  Geography  of  the  Region  about  Devil's 

Lake  and  the  Dalles  of  the   Wisconsin;    Wis.  Geol.  and  Nat. 

Hist.  Surv.,  Bull.  No.  V,  Chs.  Ill,  IV. 
SHALER  :   Rivers  and  Valleys,  in  Aspects  of  the  Earth,  pp.  143-196. 

(New  York,  1889.) 
WALCOTT  :    The  Natural  Bridge  of  Virginia,  in  Nat.  Geog.  Mag., 

Vol.  V,  pp.  59-62. 
WILLIS:     The    Northern    Appalachians,    in    Physiography    of    the 

United  States,  pp.  169-202.     (New  York,  1895.) 


CHAPTER  VI 
GLACIERS 

CHARACTERISTICS  OF  GLACIERS 

Formation  of  snow  fields  and  ice  fields.  —  When  the  water 
vapor  of  the  air  condenses  at  temperatures  below  the  freezing 
point,  it  is  usually  as  ice  crystals,  which  form  snowflakes. 
Above  an  irregular  surface  in  the  air  all  points  in  which  have  a 
temperature  of  32°  Fahrenheit  (the  isothermal  surface  of  32°), 
condensing  water  vapor  accordingly  usually  forms  crystals  of 
ice,  many  of  which  become  snowflakes,  while  below  it  the 
moisture  condenses  as  water,  and  forms  cloud  particles  or  rain- 
drops. This  surface  of  32°  Fahrenheit  is  encountered  at  vary- 
ing altitudes.  It  is  high  near  the  equator  (15,000  to  18,090 
feet  above  sea  level),  and  is  at  sea  level  in  certain  polar  regions. 
In  many  places,  as  in  northern  United  States,  for  example, 
its  position  varies  notably  with  the  seasons;  it  is  higher  in 
summer  and  lower  in  winter.  In  sufficiently  high  places  in 
low  latitudes  and  over  wide  areas  in  high  latitudes,  it  is  at  or 
near  the  surface  during  much  or  all  of  the  year.  In  such  situ- 
ations more  snow  falls  in  the  colder  months  than  is  melted  and 
evaporated  in  the  warmer  ones.  The  line  above  which  snow 
is  always  present  is  called  the  snow  line.  While  the  position 
of  the  snow  line  is  influenced  chiefly  (1)  by  temperature,  it 
varies  also  with  (2)  the  amount  of  snowfall,  being  lower  when 
the  snowfall  is  heavy  and  higher  when  it  is  light,  and  (3)  the 
character  of  the  topography,  for  some  situations  favor  the 
gathering  of  snow  and  afford  protection  against  the  sun, 
while  others  do  not.  In  general  it  does  not  depart  greatly 
from  the  summer  position  of  the  isothermal  surface  of  32°, 

191 


(192) 


GLACIERS  193 

Long-lived  accumulations  of  snow  constitute  snow  fields  (Figs. 
199  and  200). 

Snow  fields  become  ice  fields  by  the  same  processes  which 
transform  many  snow  banks  into  ice  banks  each  winter.  The 
bottom  snow  is  compressed  by  the  weight  of  that  above  and 
becomes  more  and  more  compact,  the  result  being  much  as 
when  snow  is  packed  into  an  icelike  mass  in  making  snow- 


FIG.  200.  —  Snow  fields  of  Monte  Rosa,  Switzerland.     (R.  T.  Chamberlin.) 

balls.  Water  from  rains  and  from  surface  melting  during  the 
warmer  periods  sinks  into  the  snow  beneath,  and  when  it  freezes 
helps  to  cement  the  mass.  Still  other  processes  aid  in  the 
change,  and  the  originally  loose  snow  passes  by  degrees  into 
compact  ice. 

Formation  of  glaciers.  —  When  the  ice  has  formed  in  suf- 
ficient quantity,  it  begins  to  spread  from  the  place  of  origin. 
If  formed  on  plains  or  plateaus,  ice  fields  are  thickest  at  or 
near  their  centers,  thinning  more  or  less  regularly  to  the  mar- 
gins, where  wastage  balances  snowfall.  In  such  situations  the 
ice  accordingly  moves  slowly  under  its  own  weight  in  all  direc- 
tions from  the  center.  If  formed  in  and  about  the  heads  of 
mountain  valleys,  snow  fields  and  ice  fields  acquire  a  slow 
movement  down  valley.  When  ice  fields  start  to  move,  they 
become  glaciers. 


194 


PHYSICAL  GEOLOGY 


A  glacier  spreading  in  all  di- 
rections from  its  center  on  a 
plain  or  plateau  is  an  ice  sheet 
or  ice  cap  (Fig.  212).  Glaciers 
confined  to  valleys  are  valley 
glaciers  (Figs.  201,  202,  and 
203,  and  Plate  X).  Com- 
pound glaciers  formed  on  plains 
or  plateaus  at  the  base  of  moun- 
tains by  the  union  of  valley 
glaciers  which  have  spread  out 
in  front  of  the  mouths  of  their 
mountain  valleys,  are  piedmont 
(foot  of  the  mountain)  glaciers 
(Fig.  211). 

VALLEY   GLACTERS 

Distribution  and  size. — 
There  are  hundreds  of  valley 
glaciers  among  the  mountains 
of  Alaska,  western  Canada,  and 
northwestern  United  States. 
Here  high  mountains  near  the 
coast  force  the  vapor-laden 
ocean  winds  to  precipitate 
much  moisture  in  the  form  of 
snow.  Seward  Glacier,  the 
largest  valley  glacier  in  Alaska, 
is  over  50  miles  long  and  5 
miles  and  more  wide.  Very 
few  glaciers  in  the  United 
States  are  more  than  a  mile 
long.  There  are  nearly  two 
thousand  glaciers  in  the  Alps 
Mountains.  The  longest  of 


195 


196 


PHYSICAL  GEOLOGY 


them  measures  over  10  miles,  but  the  great  majority  are  less 
than  a  mile  in  length.  They  vary  in  width  from  a  few  hundred 
feet  in  the  case  of  the  great  majority,  to  a  mile  or  more.  The 


FIG.  202.  —  Glacier  de  la  Brenva  descending  from  Mont  Blanc  on  the  Italian 
side.     (R.  T.  Chamberlin.) 

largest  ones  are  several  hundred  feet  thick.  Large  valley  gla- 
ciers occur  also  in  the  Caucasus,  Himalaya,  and  other  mountains. 
Feeding  grounds.  —  Deep  snow  fields  occupy  the  heads  of 
mountain  valleys  containing  glaciers.  Fed  by  snowfall  in  the 
valley,  by  avalanches  from  the  inclosing  slopes,  and  by  wind- 


GLACIERS  197 

swept  snow  from  the  surrounding  crags  and  peaks,  the  snow 
fields  constitute  feeding  grounds  for  the  glaciers  which  descend 
from  them.  The  larger  part  of  the  snow  of  such  fields  is 
really  granular,  half -formed  ice  (neve),  mantled  and  bordered 
with  recently  gathered  snow. 

Movement  of  glaciers.  —  Most  glaciers  move  with  extreme 
slowness.     Other  things  being  equal,  a  glacier  moves  faster 


FIG.  203.  —  Glacier  des  Grandes  Jorasses  and  the  Italian  face  of  the  Grandes 
Jorasses.     Chain  of  Mont  Blanc.     (R.  T.  Chamberlin.) 

when  it  is  thick,  when  the  slope  of  its  surface  is  considerable, 
when  its  bed  is  steep  and  regular,  and  when  its  temperature 
is  relatively  high,  than  it  does  under  the  opposite  conditions. 
The  glaciers  of  the  Alps  move  on  the  average  a  foot  or  two  a 
day,  while  some  of  the  great  glaciers  of  Alaska  and  Greenland 
move  several  times  as  fast.  Certain  Greenland  glaciers  have 
been  credited  with  the  very  unusual  rate  of  50  feet  and  more 
per  day.  From  what  has  already  been  said,  it  is  evident  that 
glaciers  move  faster  in  summer  than  in  winter.  The  ice  of  a 
glacier  also  moves  more  rapidly  in  the  center  at  the  surface,  than 
along  the  bottom  and  sides  (Why?).  Since  in  the  gathering 


198 


PHYSICAL  GEOLOGY 


FIG.  204.  —  End  of  the  Alsek  Glacier,  Alaska. 

Com  mission . ) 


(Netland,   U.S.  Boundary 


ground  of  a  glacier  the  surface  of  the  snow  and  ice  is  usually 
concave,  the  movement  is  inwards  toward  the  center,  as  well 
as  down  valley.  Farther  down  the  valley,  the  surface  is 
commonly  convex,  in  part  because  the  marginal  ice  is  melted 
faster  by  heat  reflected  from  the  walls  of  the  valley,  and  there 
is  accordingly  movement  toward  the  sides  of  the  valley,  as  well 
as  along  its  axis. 


FIG.  205.  —  The  Zwillinge  and  Grenz  Glaciers,  Switzerland.     Shows  debris 
on  the  ice,  crevasses,  etc.     (R.  T.  Chamberlin.) 


GLACIERS 


100 


The  exact  nature  of  the  movement  of  glacier  ice  is  a  mooted 
question.     The  effect,  so  far  as  the  form  of  the  glacier  is  con- 
cerned, is  much  the  same  as  in  the  movement  of  a  thick  mass  of 
tar  or  wax.     It  is  doubt- 
ful, however,  if  the  mo- 
tion is  flowage. 

Lower  limits  of  gla- 
ciers . — Glaciers  descend 
from  their  parent  snow 
fields  to  a  level  so  low 
and  so  warm  that  the 
wastage  of  the  ice  bal- 
ances its  forward  move- 


FIG.  206.  —  Muir  Glacier,  Alaska. 


ment.     Many  large  gla- 
ciers reach  far  below  the 

snow  line ;  some  of  those  in  Switzerland  end  near  grain  fields 
and  orchards.  In  high  latitudes  glaciers  may  reach  the  sea 
(Fig.  204).  Turbid  streams,  fed  by  the  melting  ice,  flow 
from  the  lower  ends  of  many  valley  glaciers  (Fig.  202) . 

Character  of  the  surface  of  valley  glaciers.  —  The  surfaces 
of  valley  glaciers  are  in  many  cases  notably  irregular  (Figs. 
205  and  206).  Varying  in  compactness,  the  surface  ice 
melts  unevenly.  Changes  in  the  slope  of  the  surface  down 
which  the  glacier  moves  cause  the  ice  to  crack  open  (Fig.  207) . 

Where  steep  or  precipi- 
tous descents  occur  in  the 
bed,  icefalls  correspond- 
ing to  waterfalls  in  rivers 
form,  and  the  ice  is  often 
shattered  by  a  multitude 

FIG.  207.  —  Portion  of  a   glacier,  showing    of  Cracks.      Great   Cracks 
crevasses  in  the  ice  due  to  changes  in  the     ,  N  ,      c  -, 

slope  of  the  bed.  (crevasses)  may  be  formed 

also   by  the  more  rapid 

motion  of  the  center  of  the  glacier,  as  compared  with  the  sides. 
One  or  more  crevasses,  often  large,  sometimes  form  where  the 
neve  of  the  lower  part  of  the  parent  snow  field  moves  away 


200 


PHYSICAL  GEOLOGY 


from  the  thinner  snows  of  the  portion  above.  This  fissure,  or 
zone  of  fissures,  where  the  glacier  proper  is  sometimes  con- 
sidered as  beginning,  is  called  the  bergschrund  (Fig.  208) .  The 


FIG.  208.  —  Bergschrund  on  east  side  of  Fremont  Peak,  Wind  River  Range, 
Wyoming.     (Baker.) 

upper  walls  of  crevasses  formed  in  these  or  other  ways,  being 
more  exposed  to  the  sun  and  weather  than  the  lower  walls, 
melt  faster,  so  that  the  openings  often  become  conspicuously 
V-shaped,  and  are  separated  by  a  complex  of  crests  and  sharp 
ridges.  Were  it  not  for  melting  follow- 
ing cracking,  most  crevasses  extending 
crosswise  of  a  glacier  would  probably 
be  closed  by  its  onward  movement. 
Rock  debris  weathered  from  the  slopes 
L^^H  above  may  accumulate  in  quantity  on 
FIG.  209.  -  Rock-capped  the  ice-  If  such  fragments  are  too 
ice  pillars.  The  rock  thick  to  be  heated  through  in  the  course 

retards  the  melting    of       /•        i          ,-,  J.T_      •       T_  0.1 

the  ice  on  which  it  rests,    of  a  day>  they  Protect  the  ice  beneath, 
and  the  melting  away   The    surrounding    ice    melting   mean- 

while>  they  COme  to  stand   On   Columns 

of  ice  (Fig.  209).  Thin  deposits  of 
earthy  matter  such,  for  example,  as  wind-deposited  dust,  have 
an  opposite  effect.  Dust  absorbs  heat  faster  than  ice  does, 
and  thin  deposits,  heating  through  readily  in  the  course  of  a 


GLACIERS 


201 


FIG.  210.  —  The  spreading  end  of  a  glacier,  Alaska.     (Brabazon.) 

day,  occasion  the  relatively  rapid  melting  of  the  ice  below. 
Depressions  known  as  dust  wells  result.  Miniature  dust  wells 
may  often  be  seen  in  rapidly  melting  snow  banks.  Summer 


FIG.  211.  —  The  Malaspina  Glacier,  and  numerous  valley  glaciers, 
right  by  Univ.  of  Wis.) 


(Copy- 


202 


PHYSICAL  GEOLOGY 


melting  of  the  surface  ice  in  the  lower  portion  of  a  glacier 
sometimes  forms  streams  which  cut  ice  valleys  in  the  glacier. 
The  above  considerations  help  to  explain  the  rough,  broken 
surfaces  of  such  glaciers  as  shown  in  Figure  206.  Travel 
across  them  is  difficult  and  often  dangerous. 

PIEDMONT    GLACIERS 

Unrestrained  by  valley  walls,  glaciers  which  extend  beyond 
the  mouths  of  their  mountain  valleys  tend  to  spread  (Fig.  210), 
and  may  come  to  occupy  a  considerable  area.  As  already 
indicated,  when  several  glaciers  descending  from  neighboring 
mountain  valleys  spread  out  along  the  base  of  the  mountains, 
they  may  unite  to  form  a  piedmont  glacier.  The  Malaspina 

Glacier  of  Alaska  is  the 
type  example  of  this  class 
(Fig.  211).  It  is  about 
1500  square  miles  in  ex- 
tent (larger  than  Rhode 
Island),  and  its  stagnant 
margin  is  covered  deeply 
with  rock  waste  which 
locally  supports  a  dense 
forest. 

ICE  SHEETS 

Some  ice  sheets  or  ice 
caps  are  rudely  circular, 
and  others  are  irregular  in 
form.  The  largest  attain 
great  size. 

South  polar  ice  sheet. 
—  Antarctic  explorers 
have  made  known  the 
existence  of  a  great  ice 
FIG.  212.  —  Map  of  Greenland  ice  sheet,  sheet  surrounding  the 


GLACIERS  203 

South  Pole.  Its  area  is  not  known,  but  it  is  believed  to  be 
more  than  3,000,000  square  miles  (about  the  size  of  the 
United  States,  exclusive , of  Alaska).  The  ice  moves  slowly 
outwards  toward  the  margins  of  the  ice  sheet,  where  great 
masses  are  detached  as  icebergs,  and  float  away. 

The  Greenland  ice  sheet.  —  Save  in  a  narrow,  rugged  coastal 
strip,  all  Greenland  is  covered  deeply  with  ice  and  snow  (Fig. 
212).  The  area  of  the  ice  is  probably  some  400,000  to  500,000 
square  miles  (seven  to  nine  times  as  large  as  the  state  of 
Illinois),  and  its  thickness  toward  the  center  more  than  a 
mile.  Occasional  mountain  tops  (called  nunataks)  rise  as 
islands  through  its  marginal  portions.  Close  to  its  edge  the 
ice  contains  many  crevasses 
and  carries  more  or  less  rock 
rubbish  on  its  surface,  but 
over  the  vast  interior  the  sur- 
face is  smooth  and  free  from 
rock  material.  Thinning  to- 
ward the  coast,  the  ice  sheet 
in  places  gives  off  great  arms, 
which  move  along  the  valleys,  FlG  213  _  An  iceberg 

often     reaching     the     ocean. 

From  the  ends  of  these  glaciers,  some  of  which  rise  as  cliffs 
200  or  300  feet  above  the  sea,  great  masses  are  detached, 
and  floated  away  as  icebergs  (Fig.  213).  Icebergs  from 
Greenland  are  carried  south  by  ocean  currents  and  winds 
to  the  latitude  of  Newfoundland,  and  sometimes  beyond. 
Rock  material  that  was  frozen  in  the  glaciers  is  carried  away 
by  the  icebergs  and  as  they  melt  it  is  dropped  on  the  ocean 
floor.  Icebergs,  however,  are  not  important  agents  of  trans- 
portation, and  most  of  what  they  carry  is  soon  dropped. 

Small  ice  caps  occur  on  various  Arctic  islands. 

Apart  from  the  geological  work  which  existing  ice  sheets  are 
doing,  and  their  climatic  and  other  influences,  they  are  inter- 
esting because  th^y  make  it  easier  to  understand  the  former 
existence  of  great  ice  sheets  in  regions  now  free  from  ice. 


204  PHYSICAL  GEOLOGY 

ANCIENT   GLACIERS 

In  much  of  Canada,  in  the  United  States  east  of  the  Mis- 
souri River  and  north  of  the  Ohio,  and  in  northern  Europe, 
the  mantle  rock  consists  of  a  mixture  of  bowlders,  gravel,  sand, 
and  clay,  ranging  in  thickness  from  a  few  inches  to  more  than 
500  feet.  These  materials  occur  separately  in  some  places, 
and  elsewhere  are  mixed  confusedly  in  all  possible  proportions. 
This  mantle  rock  was  not  produced  by  the  weathering  of  the 
underlying  rock,  for  in  any  given  locality  it  contains  material 
to  which  the  decay  of  the  bedrocks  of  that  locality  could  not 
give  rise.  This  fact  is  further  shown  by  the  contact  between  the 


FIG.  214.  — •  Diagram  showing  gradual  transition  from  residual  soil  into  the 
unaltered  rock  below.     (U.S.  Geol.  Surv.) 

mantle  rock  and  the  underlying  rock.  Mantle  rock  formed  in 
place  normally  grades  more  or  less  insensibly  into  the  firm  rock 
below  (Why?  Fig.  214).  In  the  areas  in  question,  however, 
the  surface  material  gives  place  abruptly  in  most  places  to  the 
unaltered  rock  beneath  as  suggested  in  Figures  229  and  230. 
The  mantle  rock  of  these  areas,  therefore,  was  brought  to  its 
present  position  by  one  or  more  of  "the  agents  which  transport 
materials  upon  the  land.  It  is  known  as  drift,  the  term  having 
been  applied  under  the  impression  that  it  had  been  drifted 
by  waters  to  its  present  position  from  outside  sources. 

Figure  215  shows  a  typical  exposure  of  unstratified  drift 
(till).  As  shown  in  the  illustration,  till  consists  usually  of 
material  of  many  kinds  and  sizes,  and  is  not  in  layers.  The 
stones  and  bowlders  are  sometimes  of  kinds  which  do  not 
occur  as  bedrock  within  many  miles.  Some  of  them  are 
subangular  in  form  and  have  flat  faces,  often  highly  polished 
and  covered  with  minute  scratches  (Fig.  231).  The  drift 


GLACIERS 


205 


is  often  disposed  unevenly,  so  as  to  occasion  hilly  belts  and 
undrained  depressions  (Fig.  226  and  Plate  XII).  The  trans- 
porting agent,  therefore,  gathered  its  load  from  an  area 


FIG.  215.  —  Section  of  unstratified  drift  near  Henry,  Illinois.     (Crane.) 


FIG.  216.  —  Shows  the  accumulation  of  drift  beneath  an  existing  glacier. 
Extremity  of  the  lower  Blase  Dale  Glacier  of  Disco  Island,  Greenland. 
(U.S.  Geol.Surv.) 

B.  &  B.  GEOL. 12 


206 


PHYSICAL  GEOLOGY 


large  enough  to  yield  many  different  kinds  of  rock,  and  was 
capable  of  carrying  large  bowlders  as  well  as  fine  clay,  some- 
times for  great  distances.  It  was  capable,  furthermore,  of 
giving  a  part  of  the  stones  it  carried  the  characteristics  noted 
above,  but  was  incapable  of  arranging  its  irregular  deposits 


FIG.  217.  —  Map  showing  the  areas  in  and  about  the  borders  of  North 
America  covered  by  ice  at  the  maximum  stage  of  placiation.  The  ur- 
shad">d  parts  wore  covered  by  ice;  the  dotted  portions  were  land  aro;.s 
free  from  i.-c.  (Modified  after  Willis.) 


GLACIERS 


207 


in  layers.  It  is  evident  that  the  transporting  agent  in  ques- 
tion was  neither  the  wind  nor  running  water.  The  size  of 
much  of  the  material  would  at  once  exclude  the  former, 
while  various  considerations  as  effectually  dispose  of  the 
latter.  The  largest  bowlders  of  the  till,  weighing  many  tons, 
are  far  beyond  the  transporting  power  of  common  streams. 
Streams  tend  to  round  the  stones  rolled  along  their  channels, 


FIG.  218.  —  The  shaded  area  shows  the  part  of  Europe  covered  by  the  con- 
tinental glacier  at  the  time  of  its  greatest  extent.     (James  Geikie.) 

and  are  unable  to  develop  flat  faces.  Stream-laid  beds  are 
in  layers.  The  surfaces  of  water-deposited  beds  are  without 
notable  irregularities,  such  as  often  characterize  the  till. 

Figure  216  shows  deposits  being  made  beneath  a  Green- 
land glacier  that  possess  all  the  characteristics  of  those 
shown  in  Figure  215.  So  far  as  observed,  all  the  deposits 
being  made  by  existing  glaciers  show  these  same  characteris- 
tics. Since  existing  glaciers  are  developing  exactly  the 
features  belonging  to  the  drift  of  the  great  American  and 


208  PHYSICAL  GEOLOGY 

European  areas  referred  to,  and  since  no  other  agent  is 
known  capable  of  doing  so,  it  has  been  concluded  confidently 
that  these  regions  were  formerly  covered  by  glacier  ice. 
These  glaciers  were  as  extensive  as  the  till  is  widespread, 
and  therefore  a,re  known  to  have  covered  at  their  maximum 
development  the  areas  shown  in  Figures  217  and  218.  In  a 
similar  way  other  great  areas  in  various  parts  of  the  world 
are  known  to  have  been  glaciated  at  still  earlier,  but  widely 
separated  times  (pp.  337,  394).  Some  of  these  areas  are  within 
the  tropics,  and  now  enjoy  very  warm  climates.  Glaciers  are, 
then,  one  of  the  great  geological  agents  that  have  modified 
numerous  ancient  as  well  as  present  land  surfaces.  Various 
phases  of  their  work  may  be  studied  in  most  parts  of  northern 
United  States. 

THE  GEOLOGICAL  WORK  OF  GLACIERS 

Like  winds  and  rivers,  glaciers  transport  rock  waste, 
wear  the  surfaces  over  which  they  move,  and  deposit  their 
loads  to  form  characteristic  features. 

TRANSPORTATION   AND    DEPOSITION 

As  snow  gathers  to  form  a  snow  field  it  surrounds  and 
covers  loose  pieces  of  rock  on  the  surface,  and  incloses  pro- 
jecting ledges  of  firm  rock.  When  the  snow  field  becomes 
an  ice  field,  and  begins  to  move,  it  carries  much  of  the  loose 
material  in  its  bottom  with  it,  and  may  also  break  off  and 
remove  pieces  of  the  bedrock,  so  that  the  glacier  has  a  load 
from  the  beginning.  Wherever  the  water  in  the  soil  upon 
which  the  glacier  advances  is  frozen,  it  cements  the  soil 
particles  into  a  firm  mass.  Wherever  this  ice-cemented  soil 
is  frozen  to  the  glacier  ice  above,  it  becomes,  in  effect,  a  part 
of  the  glacier,  and  is  likely  to  be  carried  on  by  its  further 
movement.  Most  of  the  material  carried  by  ice  sheets,  and 
possibly  also  much  of  that  transported  by  many  valley  gla- 
ciers, is  gathered  in  these  and  other  ways  by  the  under  sur- 


GLACIERS 


209 


face  of  the  ice.  The  material  moved  in  the  bottom  of  a 
glacier  or  lodged  beneath  it  constitutes  the  ground  moraine. 
The  ground  moraine  deposits  of  the  ancient  ice  sheets  were 


FIG.  219.  —  Diagram  to  show  how  debris  in  the  body  of  a  glacier  may  come 
to  be  on  top  through  the  lowering  of  the  surface  of  the  ice  by  melting. 

frequently  pressed  by  the  weight  of  the  overlying  ice  into  very 
dense,  compact  beds.     These  are  sometimes  called  hardpan. 

Valley  glaciers  often  carry  heavy  loads  of  rock  debris  on 
their  surfaces.  This  is  partly  material  weathered  from  the 
mountain  slopes  above,  partly  material  worn  from  elevations 
in  the  bed  of  the  glacier  and  brought  to  the  surface  by  the 
melting  of  the  ice  above  (Fig.  219),  and  partly  also  material 


FIG.  220.  —  Moraine  on  south  side  of  Hayden  Glacier,  in  west-central  Ore- 
gon. Note  the  constitution  of  the  moraine.  West  Sister  Peak,  Cascade 
Mountains,  in  background.  (Russell,  U.S.  Geol.  Sum.) 


210 


PHYSICAL  GEOLOGY 


brought  up  ! in  .-other  ways  from  the  bottom.  Belts  of  sur- 
face debris  on  the  sides  of  valley  glaciers  are  called  lateral 
moraines  (Figs.  220  and  221).  If  valley  glaciers  melt  away, 
their  surface  lateral  moraines  are  deposited  on  the  valley 
floor  beneath,  along  with  material  left  by  the  bottom  ice 
which  moved  from  the  center  to  the  sides  of  the  glacier 
(p.  198).  In  most  cases  the  latter  material  makes  up  much 


FIG.  221.  —  The  Corner  Glacier  with  its  feeders  the  Grenz,  Schwarze, 
Brei thorn,  and  Theodule  Glaciers.  Shows  lateral  and  medial  moraines, 
and  the  sources  of  the  morainic  debris.  (R.  T.  Chamberlin.) 

the  larger  part  of  the  deposits.  Lines  of  surface  debris  in 
or  near  the  center  are  medial  moraines  (Fig.  221).  In  many 
cases  medial  moraines  are  the  result  of  the  union  of  two 
valley  glaciers,  whose  adjacent  lateral  moraines  have  joined 
and  occupy  a  medial  position  on  the  main  glacier.  Debris 
on  the  surface  of  a  glacier  near  its  head  may  be  buried  by 
accumulations  of  snow  and  carried  forward  in  the  body  of 
the  ice.  Surface  material  may  also  work  its  way  through 


GLACIERS 


211 


FIG.  222.  —  Sketch  of  a  valley  glacier  in  western 
Canada,  showing  terminal  moraine. 


cracks  in  the  ice  toward  or  to  the  bottom.     On  the  other 

hand,  material  may  be  brought  in  different  ways  to  the 

surface  of  a  glacier 

from    a   position 

within   or   beneath 

the    ice,    as    noted 

above. 

Material  carried 
at  the  bottom  of  a 
glacier  may  be 
dropped  and  picked 
up  again  many 
times  before  reach- 
ing a  final  resting 
place.  Debris  may 
lodge  just  beyond 

elevations  over  which  the  ice  has  passed.  Moving  vigorously 
over  surfaces  yielding  material  readily,  the  ice  may  obtain  a 
load  which  later,  under  new  conditions,  it  cannot  carry.  At 
its  end,  the  moving  ice  is  melting  continually,  the  excess  of 
forward  movement  over  melting  being  the  measure  of  its 
advance  Material  obtained  by  the  glacier  back  from  its 
end  will  therefore,  if  not  dropped,  find  itself  sooner  or  later 
at  the  end,  where  it  will  be  deposited  as  the  inclosing  ice 
melts.  Overridden  by  the  advancing  glac  er,  it  may  be  taken 
up  once  more,  to  be  dropped  again  after  a  longer  or  shorter 
journey.  Where  the  end  of  a  valley  glacier,  or  the  edge  of 
an  ice  sheet,  remains  essentially  stationary  for  a  long  time, 
a  heavy  deposit  results  at  and  beneath  the  margin  of  the 
ice.  This  is  called  the  terminal  moraine  (Fig.  222  and  Plate 
XII).  Obviously,  the  longer  the  margin  of  the  ice  remains 
stationary,  the  larger  the  terminal  moraine  becomes.  Very 
massive  terminal  moraines  left  by  ancient  glaciers  accord- 
ingly register  very  long  stands  of  the  margin  of  the  ice.  The 
terminal  moraines  of  valley  glaciers  are  more  or  less  cresceiitic, 
the  convex  side  pointing  down  valley  (Fig.  222  ). 


212  PHYSICAL  GEOLOGY 

As  already  indicated,  glacier  deposits  are  unstratified, 
and  consist  commonly  of  materials  of  many  kinds  and  sizes. 
Ice-ground  clays  usually  retain  the  chemical  character  of 


FIG.  223.  —  Drumliri  near  McFarland,  Wis.     (Alden,  U.S.  Geol.  Surv.) 

the  parent  rocks.  Stream-borne  silts,  in  contrast,  are  gen- 
erally the  product  of  weathering,  and  therefore  differ 
chemically  from  the  rocks  from  which  they  were  derived. 
Melting  ice  has  sometimes  left  great  bowlders  in  seemingly 
insecure  positions,  forming  "  perched  bowlders,"  "  rocking 
stones,"  etc. 

In  some  places  till  has  lodged  beneath  ice  sheets  to  form 
oval  hills,  called  drumlins  (Figs.  223  and  224).  Drumlins 
vary  in  length  from  less  than  100  feet  to  more  than  a  mile, 
and  in  height  from  15  or  20  to  150  or  200  feet.  They  are 
common  in  eastern  Massachusetts,  in  parts  of  New  York  and 
Wisconsin,  and  in  some  other  localities.  In  contrast  with 


FIG.  224.  —  Drumlin  one  mile  northeast  of  Gleasondale,  Mass. 
(Alden,  U.S.  Geol.  Sun.) 

ice-worn  rock  hills  (p.  219),  the  shorter  and  steeper  slopes  of 
drumlins  generally  face  the  direction  from  which  the  ice  sheets 
came. 

The  surfaces  of  glacier  deposits  are  characteristic  (p.  205). 


PLATE  XI.  FIG.  A.  DRIFT  TOPOGRAPHY.  Contour  interval,  20  feet. 
Scale,  about  1  mile  per  inch.  (Dexter,  Michigan,  Sheet,  U.  S.  Geol.  Surv.) 

FIG.  B.  TOPOGRAPHY  DEVELOPED  BY  STREAM  EROSION.  Contour  inter- 
val, 50  feet.  Scale,  about  2  miles  per  inch.  (Lawrence,  Kansas,  Sheet, 
U.  S.  Geological  Survey.)  • 


214  PHYSICAL  GEOLOGY 

The  drift  is  usually  disposed  irregularly,  so  that  mounds 
and  hills  without  systematic  arrangement  are  associated 
with  depressions  of  varying  form  and  size,  many  of  which 

have  no  outlets.  The 
streams  of  recently  (geo- 
logically speaking)  gla- 

FIG.  225.  —  Section  of  a  lake  lying  in.  a       ciated   regions    COm- 

monly    follow     aimless 

and  roundabout  courses,  and  in  many  cases  are  interrupted  by 
lakes  and  marshes  (Plate  XI,  Fig.  A).  All  this  is  in  contrast 
with  topographies  due  to  river  erosion.  Since  such  topog- 
raphies have  resulted  from  the  cutting  of  valleys,  the  ele- 
vations are  distributed  systematically  with  reference  to  the 
depressions,  all  of  which  have  outlets  (Plate  XI,  Fig.  B). 
As  we  have  already  seen  (p.  207),  it  is  in  contrast,  too,  with 
topographies  due  to  stream  deposition.  The  lake  basins 
and  other  surface  hollows  of  drift  areas  have  been  formed 
in  several  ways.  Some  are  sections  of  preglacial  river 
valleys  in  which  drift  was  deposited  unevenly.  Where  the 
ice  deposited  more  material  around  than  on  a  given  area, 
the  latter  came  to  stand  lower  than  its  surroundings  (Fig. 
225).  Still  other  basins  were  gouged  out  of  the  underlying 
rocks  by  the  ice.  The  thousands  of  lakes  in  the  northern 
part  of  the  United  States  are  practically  all  of  glacial 
origin. 

The  features  described  above  as  distinctive  of  drift  sur- 
faces are  most  pronounced  in  terminal  moraines,  which  are 
often  characterized  by  notably  hummocky  topography  (Figs. 
226  and  227,  and  Plate  XII).  Numerous  mounds,  hillocks, 
and  short  ridges,  ranging  in  diameter  from  a  few  feet  to  a 
half  mile  and  more,  and  reaching  occasionally  a  height  of 
100  to  200  feet,  are  associated  with  depressions  varying  in 
depth  from  inches  to  scores  of  feet,  and  in  area  ranging  up 
to  many  acres.  Many  of  the  depressions  contain  ponds 
or  lakes.  Elevations  and  depressions  are  huddled  together 
in  confusion.  Ground  moraine  surfaces  are  usually  less 


215 


PLATE  XII.  TERMINAL  MORAINE  AND  OUTWASH  PLAIN.  Contour  in- 
terval, 20  feet.  Scale,  about  1  mile  per  inch.  (St.  Croix  Dalles,  Wisconsin- 
Minnesota,  Sheet,  U.  S.  Geological  Survey.) 


216 


PHYSICAL  GEOLOGY 


irregular.  Hollows  are  not  so  deep,  swells  are  not  so  high, 
and  slopes  are  gentler.  Certain  ground  moraine  drift  plains 
are  almost  flat. 


FIG.  226.  —  Terminal  moraine  topography  six  miles  southwest  of    Glen- 
buelah,  Sheboygan  Co.,  Wis.     (Alden,  'U.S.  Geol.  Surv.) 

The   deposition   of   drift   may  render   a  surface   rougher 
than  before  (Fig.  228),  or  may  reduce  the  relief  (Fig.  229). 


FIG.  227.  —  Terminal  moraine  topography  near  Oconomowoc,   Wis.     The 
elevations  are  kames.     (Alden,  U.S.  Geol.  Surv.) 


GLACIERS 


217 


The  latter  seems  to  have  been  the  result  over  most  of  the 
lake  and  prairie  plains  in  northern  United  States. 


FIG.  228.  —  Diagram  showing  how  a  nearly 
level  surface  may  be  replaced  by  a  rough 
one  through  the  uneven  deposition  of 
drift. 


TOPOGRAPHIC  FEATURES  DEVELOPED  BY  GLACIER  EROSION 

How  glaciers  erode.  —  Since  it  is  much  softer  than  rock, 
pure  ice  accomplishes  little  or  no  wear  upon  smooth,  firm 
surfaces ;  rather  is  it  worn  by  the  harder  rock.  As  already 
indicated,  however,  the 
bottom  ice  is  likely  to  be 
charged  with  rock  frag- 
ments, and  thus  armed, 
glaciers  become  efficient 
agents  of  erosion.  Their 
rock  tools  are  pressed 
with  tremendous  force 
upon  the  surfaces  over 
and  against  which  they 
move,  and  each  kind 
does  its  appropriate  work. 
Clay  particles  tend  to 
smooth  and  polish,  sand 
grains  and  hard  pebbles 
to  scratch  (striate),  and 
bowlders  to  gouge  and 
groove  the  bedrock  (Fig. 
230).  Meanwhile,  the 
tools  are  themselves  worn.  The  weaker  ones  may  be  ground 
into  fine  bits,  even  to  rock  flour.  The  stronger  ones  often  are 
marked  typically ;  their  sides  are  worn  flat,  and,  like  the  bed- 
rock, are  polished  by  clay  and  striated  by  sand  (Fig.  231). 

Thick  ice  moving  over  much-jointed  surfaces  sometimes 
quarries  out  blocks  of  rock  by  a  process  known  as  plucking. 
The  bottom  ice  is  pressed  by  the  great  weight  of  that  above 
into  the  joints,  bedding  planes,  and  other  openings  of  the 
rocks,  and  as  the  glacier  moves  onward,  fragments,  some- 


FIG.  229.  —  Diagram  showing  how  glacial 
drift  may  be  so  disposed  as  to  replace 
a  hilly  surface  with  a  comparatively  level 
one. 


218 


PHYSICAL  GEOLOGY 


FIG.  230.  —  Glaciated  rock  surface.     The  view  shows  also  the  relation  of 
drift  to  the  bedrock  beneath.     Northern  Ohio.     (Stauffer.) 

times  of  considerable  size,  are  dislodged.  The  freezing  of 
water  in  the  openings  of  the  rocks  beneath  the  ice  helps  in 
the  process. 

General  effect  of  erosion  upon  relief.  —  Other  things 
equal,  ice  sheets  erode  most  in  regions  where  many  slopes 
oppose  the  advance  of  the  ice.  In  flat  regions  the  frozen 
mantle  rock  has  sometimes  been  overridden  by  thick  ice, 
and  little  disturbed.  In  rugged  regions  ice  sheets  tend  to 


FIG.  231.  —  Glaciated  stones. 


GLACIERS 


219 


plane  away  the  angularities  of  the  surface,  reducing  and 

smoothing  the  slopes.     Where  hilltops  are  worn,  the  tend- 

ency is  to  reduce  the  relief.     Where  glaciers  move  along  the 

axes  of  valleys,  they  tend 

to  widen  and  deepen  them, 

and  so  to  increase  the  re- 

lief. 

Ice-worn  hills  and  ba- 
sins. —  Hills  that  have 
been  eroded  vigorously  by 
ice  sheets  are  usually  of 
characteristic  form  (Fig. 

232).    The  Side  against  and    FIG.  232.  —  Lamberts  Dome.     A  glaci- 
,  .   ,      ,  ,        .  ,        ated  hill  of  granite.     Upper  Tuolumne 

up  which   the   ice   moved      River>    (Fairbanks.) 
(the    stoss    side)    suffered 

most  wear,  and  was  lengthened  and  smoothed.  The  side 
away  from  and  down  which  the  ice  moved  (the  lee  side)  is 
commonly  the  shorter  and  steeper,  and  was  sometimes  left 
rough  and  irregular  by  plucking.  Where  it  crosses  valleys 
and  basins,  and  erodes  them,  an  ice  sheet  usually  wears  chiefly 
the  sides  opposed  to  its  advance,  making  them  gentler  and 
smoother  (Fig.  233). 

The  shapes  of  glaciated  rock  hills  and  basins,  then,  record 
the  direction  of  movement  of  ancient  glaciers,  the  longer 
and  smoother  slopes  facing  the  direction  whence  the  ice 

came.     Since  minute  projections 
and    depressions     are     similarly 

the  ice>   the 


FIG.    233.  -  Diagram    showing  shaPed 

change  which  may  be  made  in  tion  of  any  Small  Surface  of  glaci- 

the  cross  section  of  a  valley  by  t  d  bedrock  wiU  usually  snOW  the 

an  ice  sheet  which  moves  across  J 

it.    Dotted  line  shows  side  of  direction  in  which  the  ice  moved. 

valley  before  glaciation.  (How   much    CQuld    be    toid    CQn. 

cerning  the   direction  of    movement  by  the  trend  of  the 
striae?) 

Ice-shaped  valleys.  —  Valley  glaciers  tend  by  erosion  to 
widen  and  deepen  their  valleys  and  to  steepen  and  smooth 


220 


PLATE  XIII.  A  PORTION  OF  THE  SIERRA  NEVADA  MOUNTAINS,  SHOW- 
ING GLACIATED  VALLEYS.  Contour  interval,  100  feet.  Scale,  about  2  miles 
per  inch.  (Mt.  Whitney,  California,  Sheet,  U.  S.  Geological  Survey.) 


GLACIERS 


221 


their  sides.  Thus  V-shaped  valleys  are  changed  to  U- 
shaped  troughs  (Figs.  234  and  235,  Plate  XIII).  The  en- 
larged heads  of  glaciated  valleys  have  broad  bottoms,  often 
containing  ice-worn  rock  basins,  and  high,  precipitous  walls. 
Such  valley  heads  are  called  cirques  (Figs.  236  and  237,  Plate 
XIII).  In  winter  the 
neve  and  ice  of  the 
upper  glacier  freezes 
to  the  valley  walls. 
In  spring,  the  ice 
pulls  away  from  them 
and  dislodges  and 
carries  with  it  many 
rock  fragments. 
During  the  summer 
the  walls  of  the  val- 
ley head  may  be 
more  or  less  exposed 
to  the  agents  of 
weathering,  and  ma- 
terial prepared  for 
later  removal  by  the 
ice.  This  process 
helps  to  drive  the 
sides  and  head  of 
the  valley  back  into 
steep  cliffs. 

Lakes  dot  the  bot- 
toms of  most  glaci- 


FIG.  234.  —  Glacial  trough  near  Green  River 
lakes,  Wind  River  Range,  Wyoming.  Shows 
contrast  between  glaciated  topography  be- 
low, and  unglaciated  topography  above. 
The  lake  in  the  foreground  is  held  in  by  a 
morainic  dam.  (Baker.) 


ated  valleys  (Fig. 
238,  Plates  X  and  XIII).  Some  of  them  occupy  rock  basins 
gouged  out  by  the  glacier  (Fig.  239),  and  others  fill  depres- 
sions on  the  up-valley  sides  of  morainic  dams  (Fig.  240). 

Tributary  valleys  normally  join  their  main  valleys  at  even 
grade.  But  main  valleys  are  often  deepened  by  glaciers 
more  than  their  tributaries.  Because  of  this,  and  because 


222 


PHYSICAL  GEOLOGY 


FIG.  235.  —  Glacial  trough  with  hanging  valleys. 
River.     (Baker.) 


Upper  canon  of  Green 


of  the  widening  of  the  bottoms  of  the  main  valleys,  the 
floors  of  the  tributary  valleys  at  their  mouths  are  left  stand- 
ing higher  (sometimes  1000  feet  or  more)  than  the  opposite 

bottoms  of  the  main  val- 
leys. After  the  disap- 
pearance of  the  ice,  the 
streams  of  the  tributary 
valleys  descend  in  rapids 
or  falls  to  the  main 
streams.  Such  elevated 
tributary  valleys  are 
known  as  hanging  valleys 
(Figs.  241  and  235).  The 
same  condition  is  of 
course  brought  about 
where  a  main  valley  is 


FIG.  236.  —  View  in  the  Bighorn  Moun- 
tains, Wyo.  The  cirque  in  the  back- 
ground contains  Cloud  Peak  Glacier, 
which  has  a  length  of  nearly  a  mile. 
The  cirque  walls  are  in  places  about 
1500  feet  in  height.  (Trowbridge.) 


glaciated,  while  its  tribu- 
taries remain  free  from 
ice. 

The    topographic    fea- 


GLACIERS  223 

tures  described  above  occur  in  western  United  States  and  Can- 
ada, among  the  Alps,  and  elsewhere,  in  many  valleys  now  ice- 
free.  They  have  been  more  or  less  modified,  however,  since 


FIG.  237.  —  Near  view  of  the  walls  of  a  cirque. 

the  disappearance  of  the  glaciers,  and  will  ultimately  be  de- 
stroyed. The  glaciated  rock  surfaces  not  covered  with  drift 
are  being  weathered.  The  steep  sides  and  heads  of  the  val- 
leys favor  landslides,  the  accumulation  of  talus,  and  the 
formation  of  alluvial  cones.  The  streams  are  grading  the  of- 
ten irregular  beds  of  the  former  glaciers,  lowering  the  hanging 
valleys,  and  filling  or  draining  the  lakes.  The  relative  extent 
of  these  changes  in  different  valleys  is  a  rough  measure  of  the 


224 


PHYSICAL  GEOLOGY 


relative  amount  of 
time  which  has  elapsed 
since  the  glaciers 
melted  away. 

Fiords.  —  Where 
thick  glaciers  push  into 
the  sea  through  nar- 
row bays,  they  may 
scour  the  bay  bottoms 
much  deeper,  and  at 
the  same  time  wear  the 
bay  heads  back  into 
the  mainland.  Where 
ancient  glaciers  have 
disappeared  from  such 
bays,  the  sea  has  en- 
tered to  form  long, 
narrow,  steep-walled 
embayments,  called 
fiords  (Figs.  242  and 
243).  Typical  fiords 
abound  along  the 
Norwegian,  Alaskan  (Fig.  244),  and  certain  other  high-lati- 
tude coasts.  In  most  cases  their  depth  is  due  partly  to  sub- 
mergence of  the  coast.  Many  islands  fringe  these  shores, 
representing  for  the  most  part  higher 
land  whose  lower  surroundings  were 
drowned. 


FIG.  238.  —  Lakes  of  glacial  origin  in  a  moun- 
tain valley.  The  nearest  lake  is  in  an  ice- 
scoured  rock  basin ;  the  others  are  held  in 
by  drift.  Note  the  U-shaped  cross  section 
of  the  valley  in  the  middle  distance.  Piney 
Creek  Valley,  Bighorn  Mountains.  (Trow- 
bridge.) 


FIG.  240.  —  Section  of  a  lake  behind  a 
barrier  of  drift. 


FIG.  239.  —  Section  of 
a  lake  lying  in  an  ice- 
scoured  rock  basin. 

Which  way  did  the 
glacier  move  which 
formed  this  basin  ? 


GLACIERS  225 


THE  WORK  OF  WATERS  ASSOCIATED  WITH  GLACIERS 

Water   from   the   surface   melting   of   summer   and   from 
rains  sometimes  forms  streams  that  flow  in  valleys  which 


FIG.  241.  —  Hanging  valleys,  Lyngen  Fiord,  Norway.     The  hanging  valley 
in  the  center  contains  a  glacier.     (R.  T.  Chamberlin.) 

they  have  cut  in  the  ice  (p.  202).     Water  also  finds  its  way 
through  cracks  and  crevasses  to  the  bottom  to  form  sub- 


Fio.  242.  —  Troldfjord,  Lofoten  Islands,  coast  of  Norway.     (R.  T.   Cham- 
berlin.) 


226 


PHYSICAL  GEOLOGY 


FIG.  243.  —  Fiord  at  North  Cape,  Norway.     Photograph  taken  at    12.08 
A.M.,  July  7,  1909.     (R.  T.  Chamberlin.) 

glacial  streams.  Subglacial  waters  are  formed,  too,  by  the 
melting  of  the  bottom  ice  because  of  friction  between  the 
glacier  and  its  bed,  and  in  other  ways.  Ice-fed  streams,  in 

most  cases  heavily 
charged  with  gravel, 
sand,  and  silt,  flow 
from  the  ends  of  val- 
ley glaciers,  and  at 
many  points  from 
the  edges  of  ice 
sheets. 

The  streams  be- 
neath glaciers  and 
beyond  their  ends 
and  edges  are  com- 

tlPHi  "      181P$;  monlv     {i.u^Tadintf 

rather  than  degrad- 
ing  streams.  There- 
fore the  deposits 
made  by  glacial 

152*  op  i3o-          waters  are  the   only 

FIG.  244.  —  Alaskan  fiords.,  7  matters    in    connec- 


GLACIERS 


227 


tion  with  their  work  which  need  be  discussed.  Like  other 
stream-laid  beds,  such  deposits  are  in  layers  and  consequently 
unlike  the  till  deposited  directly  by  the  ice. 


FIG.  245.  —  Diagram  to  illustrate  the  building  of  a  valley  train. 
Describe  and  account  for  what  you  see  along  the  front  of  the  ice  sheet. 

Valley  trains.  —  Streams  flowing  away  from  glaciers  in 
valleys  of  moderate  slope  are  generally  overloaded  with 
debris  derived  from  the  ice  and  washed  from  tributary  slopes 
beyond  the  ice.  They  therefore  make  deposits  along  their 
braided  channels,  building  river  plains  of  sand  and  gravel. 
Such  aggradational  plains  are  valley  trains  (Fig.  245).  The 
stream  deposits  more  and  coarser  material  near  the  ice,  and 
less  and  finer  sediment  farther  from  it.  The  downstream 
slope  of  valley  trains  is  accordingly  steepest  near  the  ice 
and  increasingly  gentle  away  from  it  (Fig.  246).  Much  of 
the  material  of  valley  trains  is  cross-bedded. 

Many  remnants  of  valley  trains,  in  the  form  of  terraces, 
occur  along  the  rivers  of  northern  and  northeastern  United 
States.  The  longer  the  edge  of  the  ancient  ice  sheet  from 


FIG.  246.  —  Diagram  of  a  valley  train,  showing  the  slope  of  its  surface,  its 
structure,  and  its  relation  to  the  terminal  moraine  in  which  it  heads. 

which  the  aggrading  streams  issued  remained  stationary, 
the  greater  the  valley  fillings.  Heavy  valley  train  deposits, 
like  massive  terminal  moraines,  therefore  indicate  protracted 
stands  of  the  edge  of  the  ice. 


228 


PHYSICAL  GEOLOGY 


Outwash  plains.  —  Where  overloaded  streams  that   issued 
from  the  ancient  ice  sheet  did  not  find  valleys  for  their  ac- 
commodation, as  was  often  the  case, 
they  spread  their  material  in  fanlike 
deposits  in   front  of  the  ice.     Many 
FIG.    247. —  Section  of  a    such   deposits   made   by  neighboring 
lake  at  the  margin  of  an    streams  often  joined  to  form  alluvial 

ice  sheet.  .  ,77- 

plains,    known    as     outwash    plains, 

which  slope  gently  away  from  the  terminal  moraines  which 
they  front  (Plate  XII). 

Deltas.  —  Marginal  lakes  were  sometimes  formed  at  the 
edge  of  the  ancient  ice  sheet  where  the  land  sloped  down- 
wards toward  the  ice,  forming  a  temporary  basin  (Fig.  247). 
Where  streams  issued  from  the  ice  at  the  edges  of  lakes, 
they  deposited  their  loads  in  the  form  of  deltas.  Such  deltas 
are  common  in  parts  of  New  England. 

Kames.  —  The  edge  of  the  ancient  ice  sheet  was  doubt- 
less jagged  and  irregular  (Why?).  Subglacial  streams, 
flowing  in  tunnels  beneath  the  ice,  were  often  under  great 
pressure,  like  the  water  in  a  long  tube.  When  such  streams 
issued  from  beneath  the  ice  in  reentrant  angles  of  its  edge, 
the  pressure,  and  therefore  their  velocity  and  carrying  power, 
were  reduced.  This  caused  them  to  make  deposits,  which 
were  shaped  by  the  par- 
tially inclosing  ice  walls. 
In  this  way  irregular 
mounds  and  hillocks  of 
rudely  stratified  and 
water-worn  material  were 
formed  in  association  with 
the  unstratified  deposits  of 
the  terminal  moraine. 
Such  deposits  are  called 
kames  (Figs.  248  and  227). 

Eskers.  —  Subglacial  streams  sometimes  deposited  sand 
and  gravel  along  the  floors  of  the  ice  tunnels  through  which 


FIG.  248.  —  Kame  east  of  Kewaskum, 
Dodge  County,  Wis.  (Alden,  U.S.  Geol. 
Surv.) 


GLACIERS 


229 


FIG.  249. — Bridgewater  Esker,  Rice  County,  Minn.     (R.  T.  Chamberlin.) 

they  flowed.  On  the  melting  of  the  ice  these  deposits  re- 
mained as  serpentine  ridges,  called  eskers  (Fig.  249).  Like 
the  material  of  kames,  that  of  eskers  is  usually  rounded  and 
poorly  stratified  (Fig.  250). 

The  bulk  of  the  stratified  material  of  the  ancient  drift 
sheets  does  not  form  distinct  topographic  features,  but  is 
scattered  in  irregular  belts  and  layers  within  and  beneath 
the  till,  as  well  as  upon  it. 


FIG.  250.  —  Section  of  an  esker  near  Randolph,  Wis.,  showing  its  com- 
position and  structure.  Many  eskers  are  composed  of  much  coarser  ma- 
terial. (Miller.) 

B.  &  B.  GEOL. 13 


230  PHYSICAL  GEOLOGY 

SUMMARY 

A  chief  function  of  glaciers  is  to  return  to  lower  and  warmer 
levels  moisture  which  otherwise  would  be  imprisoned  in- 
definitely as  snow  and  ice.  Geologically,  glaciers,  like  rivers, 
have  as  their  principal  mission  the  wearing  of  the  land  and 
the  moving  of  the  waste  toward  the  sea.  In  the  aggregate, 
however,  they  are  much  less  important  agents  of  change 
than  rivers.  Streams  are,  and  since  the  very  early  history 
of  the  earth  have  always  been,  at  work  nearly  everywhere 
upon  the  land.  Even  in  deserts  there  are  very  few  large 
areas  without  valleys,  although  such  valleys  may  be  occu- 
pied only  by  temporary  streams.  At  present,  glaciers  affect 
but  a  small  fraction  of  the  land  surface,  and  while,  as  we 
have  seen,  their  extent  has  been  much  greater  than  now  at 
various  times  in  the  past,  this  was  true,  so  far  as  known, 
for  only  comparatively  short  periods.  Glaciers  are  at  a 
disadvantage,  too,  from  the  fact  that  their  work  is  entirely 
mechanical.  On  the  other  hand,  their  activities  are  not  so 
conditioned  by  the  hardness  and  structure  of  the  surfaces 
upon  which  they  work  as  are  those  of  streams. 

Although  not  so  important  geologically  as  they,  ice  takes 
its  place  with  air  and  water  as  one  of  the  three  great  grada- 
tional  agents  which  modify  land  surfaces. 

QUESTIONS 

1.  Why  is  the  snow  line  much  lower  on  the  southern    (sunny) 
side  of  the  Himalaya  Mountains  than  on  the  northern  (shady  and 
cooler)  side? 

2.  Why  have  the  Sierra  Nevada  and  Cascade  Mountains  more 
glaciers  than  the  Rocky  Mountains  ?     Why  are  there  more  in  the 
northern  than  in  the  southern  Rockies  ? 

3.  What  are  the  factors  upon  which  the  size  of  a  given  valley 
glacier  will  depend  ? 

4.  What  things  limit  the  height  which  rock-capped  ice   pillars 
such  as  those  shown  in  Figure  209  may  attain  ? 

5.  Do  all  parts  of  the  medial  surface  line  of  a  valley  glacier 
move  at  the  same  rate?     Why? 


GLACIERS  231 

6.  In  what  part  of  a  valley  glacier  should  erosion  be  greatest  ? 
Least  ?     Why  ? 

7.  (1)  What  will  be  the  effect  of  the  slow  degradation  of  glacier- 
bearing   mountains   upon   the   snowfall   they  receive?     (2)    What 
influence  will  this  have  upon  the  position  of  the  snow  line  ?     (3)  How 
will  the  facts  involved  in  the  two  preceding  questions  affect  the 
size  and  length  of  the  glaciers  ?     (4)  When  will  the  mountains  cease 
to  have  glaciers  ? 

8.  How  could  one  determine  in  the  field  the  approximate  thick- 
ness of  the  glaciers  which  formerly  occupied  the  valleys  shown  in 
Figures  234  and  235  ? 

9.  What  conditions  would  produce  valley  trains   (1)   of  high, 
and  (2)  of  low  average  gradient  ? 

10.  Why  are  eskers   usually  roughly  parallel  with  the  direction 
of  ice  movement  ? 

11.  Compare  and  contrast   typical  topographies  due  to  river 
erosion  and  to  glaciation. 

12.  Moraine  topography  and  dune  topography  are  sometimes 
similar.     How  might  the  two  be  distinguished  in  the  field  ? 

REFERENCES 

ALDEN  :    Drumlins    of    Southeastern    Wisconsin;    Bull.    273,   U.S. 

Geol.  Surv. 
BRIGHAM  :    The  Fiords  of  Norway,  in  Bull.  Am.  Geog.  Soc.,  Vol. 

XXXVIII,  pp.  337-348. 
CHAMBERLIN  :    The  Rock-Scorings  of  the  Great  Ice  Invasions,  in  7th 

Ann.  Kept.,  U.S.  Geol.  Surv.,  pp.  147-248. 
DAVIS  :  Glacial  Erosion  in  France,  Switzerland  and  Norway,  in  Proc. 

Bost.  Soc.  Nat.  Hist.,  Vol.  XXIX,  pp.  273-322. 
-  Hanging  Valleys,  in  Science,  N.  S.,  Vol.  XXV,  pp.  833-836. 
GANNETT  :  Lake  Chelan,  in  Nat.  Geog.  Mag.,  Vol.  IX,  pp.  417-428. 
GEIKIE,  J.  :   Land-forms  modified  by  Glacial  Action,  in  Earth  Sculp- 
ture, Chs.  X,  XI.     (New  York,  1898.) 
GEIKIE,  SIR  A.  :   Scenery  of  Scotland,  Chs.  IV,  X,  XI,  XIV,  XVII. 

3d  ed.     (London,  1901.) 
GILBERT  :    Alaska;    Glaciers  and  Glaciation.     Vol.  Ill  of  Harriman 

Alaska  Expedition.     (New  York,  1904.) 

NANSEN  :    The  First  Crossing  of  Greenland.     (London,  1893.) 
PEARY:     Northward  over  the  ^  Great   Ice."     2  vols.     (New  York, 

1898.) 
RUSSELL  :    Glaciers  of  North  America.     (Boston,  1897.) 

—  Glaciers  of   Mount    Rainier,  in    18th  Ann.  Rept.,  U.S.  Geol. 

Surv.,  Pt.  II,  pp.  349-415. 


232  PHYSICAL  GEOLOGY 

Malaspina  Glacier,  in  Jour,  of  Geol.,  Vol.  I,  pp.  219-245. 

Glaciers  of  the  St.  Elias  Region,  in  Nat.  Geog.  Mag.,  Vol.  Ill, 

pp.  176-188. 
SALISBURY  :    Salient  Points  Concerning  the  Glacial  Geology  of  North 

Greenland,  in  Jour,  of  Geol.,  Vol.  IV,  pp.  769-810. 

-  The  Drift,  in  Jour,  of  Geol.,  Vol.  II,  pp.  708-724,  837-851 ; 
Vol.  Ill,  pp.  70-97. 

-  The  Glacial  Geology  of  New  Jersey ;   N.J.  Geol.  Surv.,  Vol.  V. 
SALISBURY  AND  ATWOOD  :    The  Glacial  Period,  in  Bull.  No.  V,  Wis. 

Geol.  and  Nat.  Hist.  Surv.,  Ch.  V. 
SHALER  :    Glaciers,  in  Outlines  of  the  Earth's   History,   Ch.  VI. 

(New  York,  1898.) 
SHALER  AND  DAVIS  :    Illustrations  of  the  Earth's  Surface;    Glaciers. 

(Boston,  1881.) 
TYNDALL  :   The  Glaciers  of  the  Alps.     (London,  1860.) 


CHAPTER  VII 
OCEANS  AND  LAKES 

Oceans  and  ocean  basins.  —  The  oceans  have  an  area 
(143,000,000  square  miles)  nearly  three  times  as  great  as 
that  of  the  lands  (54,000,000  square  miles).  They  cover 
the  low  edges  of  the  continents,  so  that  their  area  is  greater 
(by  some  10,000,000  square  miles)  than  that  of  the  ocean 
basins.  The  ocean  basins  are  a  little  more  than  twice  as 
extensive  as  the  continental  plateaus.  The  submerged  edges 
of  the  continental  blocks  are  called  the  continental  shelves 
(Fig.  251).  The  shallow  seas  on  the  continental  shelves 
may  be  thought  of  as  remnants  of  the  vast,  shallow  seas 
which  at  various  times  in  the  past  covered  large  portions  of 
the  continents. 


Continental 


5ea  level 


FIG.  251.  —  Diagram  showing  a  continental  shelf,  and  its  relation  to  the 
land  on  one  side  and  to  an  ocean  basin  on  the  other. 

Soundings  have  shown  that  the  bottoms  of  the  ocean 
basins  are  generally  smooth.  Mountain  chains  and  plateau- 
like  swells  are  not  altogether  wanting,  while  great  volcanic 
cones  are  numerous  in  parts  of  the  Pacific  Ocean,  many  of 
them  rising  as  mountainous  islands  thousands  of  feet  above 
the  level  of  the  sea.  There  are  also  submarine  fault  scarps 
and  relatively  small  areas  much  lower  than  the  surrounding 
ocean  floor,  called  deeps.  Nevertheless,  these  features  occupy 
but  a  small  fraction  of  the  ocean  bottom,  nine  tenths  or 

233 


234  PHYSICAL  GEOLOGY 

more  of  which  forms  a  monotonously  flat  plain.  As  already 
indicated  (p.  62),  the  absence  of  the  familiar  hills,  valleys, 
and  many  other  features  of  the  land  is  due  (1)  to  the  fact 
that  the  bottoms  of  the  ocean  basins  are  protected  from  the 
attack  of  wind  and  weather,  of  streams  and  of  glaciers,  the 
agents  which  sculpture  land  surfaces,  and  (2)  to  the  effects 
of  the  deposition  of  sediment  in  the  ocean. 

The  average  depth  of  the  ocean  basins  is  a  little  less  than 
two  and  one  half  miles  (about  13,000  feet).  This  is  nearly 
six  times  the  average  elevation  (some  2300  feet)  of  the  lands 
above  sea  level. 

Offices  of  the  ocean.  —  (1)  Nearly  all  the  moisture  which 
is  condensed  upon  the  surface  of  the  land  as  rain  or  snow, 
or  in  less  important  forms,  comes  directly  or  indirectly  from 
the  ocean.  Together  with  the  atmosphere  (p.  86),  the 
ocean  therefore  makes  possible  the  work  of  streams,  of  ground 
water,  and  of  glaciers.  Without  the  moisture  which  is 
evaporated  from  the  ocean  and  carried  by  the  winds  to  be 
precipitated  over  the  land,  neither  plant  nor  animal  life 
could  flourish.  This  constitutes  perhaps  the  greatest  service 
which  the  ocean  renders. 

(2)  The  ocean  tends  to  regulate  the  distribution  of  tem- 
perature over  the  earth's  surface.     The  temperature  of  the 
winds  is  modified  by  that  of  the  ocean  surfaces  across  which 
they  blow,  and  the  heat  or  cold  gained  is  carried  over  the 
land  for  greater  or  lesser  distances.     Warm  ocean  currents 
from  low  latitudes  carry  great  quantities  of  heat  poleward. 
Cold  currents  from  high  latitudes  carry  lower  temperatures 
equatorward.     Just  as  oceanic   islands   have  more  uniform 
climates  than  great  land  masses,  so  in  past  ages  widespread 
invasions  of  the  lands  by  the  sea  resulted  in  periods  of  uni- 
form (oceanic)   climate,  while  great  extensions  of  the  land 
areas  coincided  with  periods  of  variable  (continental)  climate. 

(3)  In  preceding  chapters  it  has  been  pointed  out  that 
the  ocean  is  the  ultimate  goal  of  all  the  waste  of  the  land, 
which  is  spread  out  upon  its  floor  as  layers  of  sediment. 


OCEANS  AND  LAKES  235 

Throughout  the  geological  ages  a  chief  service  of  the  ocean 
has  been  to  receive,  arrange,  and  preserve  the  materials 
from  which  new  land  areas  were  later  formed.  While  aggra- 
dation has  always  been  the  dominant  gradational  process  in 
the  ocean,  degradation  has  always  been  of  chief  importance 
upon  the  land. 

(4)  Finally,  the  sea  has  always  been  engaged  in  eroding 
portions  of  its  shores.  Thus  it  tends  persistently  to  reduce 
the  area  of  the  land,  and  to  increase  its  own  extent. 

The  movements  of  sea  waters.  —  The  geologically  im- 
portant movements  of  the  sea  are  wind  waves,  currents,  and 
tidal  waves.  Earthquake  waves  and  certain  other  occa- 
sional and  unusual  movements  are  at  times  important. 

Wind  waves  are  caused  by  the  pressure  of  the  wind  upon 
the  surface  of  the  water.  In  the  open  sea,  the  water  is 
pushed  forward  very  little  and  slowly,  even  though  the 
wave  form  advances  with  rapidity.  Each  particle  moves 
through  an  elliptical  path  every  time  that  a  wave  passes,  but 
returns  essentially  to  the  point  of  starting.  The  movement 
of  the  water  particles  in  a  wave  has  been  likened  frequently 
to  that  in  a  field  of  tall  grass  across  which  the  wind  is  blow- 
ing. Each  blade  is  bent  up  and  down,  back  and  forth,  yet 
retains  its  place.  Waves  are  propagated  with  gradually 
lessening  height  far  beyond  the  area  of  the  storm  which 
generated  them;  here  the  diminishing  waves  are  called 
swells. 

On  approaching  land,  waves  drag  bottom  and  the  oscilla- 
tory movement  passes  into  a  true  onward  movement.  The 
unimpeded  top  of  the  wave  moves  faster  than  the  lower 
part,  which  is  retarded  by  friction  with  the  bottom,  and  the 
front  of  the  wave  accordingly  becomes  increasingly  steep, 
until  the  crest  topples  over  and  the  wave  breaks  with  all 
its  weight  upon  the  shallow  bottom  or  upon  the  shore  line 
(Fig.  252).  The  water  of  the  broken  wave  rushes  up  the 
beach,  and  then  returns  seaward  under  gravity,  forming  the 
undertow. 


236  PHYSICAL  GEOLOGY 

The  great  ocean  currents  are  caused  primarily  by  the 
winds.  Their  courses  are  determined  (1)  by  the  direction 
of  the  winds,  (2)  by  the  arrangement  of  the  land  masses, 
(3)  locally,  by  the  configuration  of  the  ocean  bottom,  and  (4) 
by  the  earth's  rotation,  which  deflects  them  toward  the  right 
hand  in  the  northern  hemisphere,  and  toward  the  left  hand 
in  the  southern  hemisphere.  The  importance  of  ocean  cur- 
rents in  connection  with  the  distribution  of  temperature 
has  been  referred  to.  Warm  and  cold  currents  influence 
greatly  the  present  distribu- 
tion of  marine  life,  and  the 
ocean  currents  of  earlier  geo- 
logical periods,  some  of  which 
flowed  across  the  centers  of 
the  continents  (then  sub- 
merged), have  to  be  taken  into 
account  in  explaining  the  dis- 
FIG.  252.  —  Breakers  on  the  coast  tribution  of  former  life.  The 

(kirbaanksn)ia  **  ***'  BUCh°n'  mechanical  work  of  ocean  cur- 
rents is  in  general  unimpor- 
tant. Deep  currents  in  shallow  places  may  scour  the  ocean 
bottom,  but  the  bottoms  of  currents  are  usually  far  above 
that  of  the  sea.  Certain  ocean  currents  carry  away  sediment 
brought  to  them  by  the  streams  of  the  neighboring  land,  but 
large  quantities  are  never  carried  far.  (Why  are  ocean  cur- 
rents not  so  efficient  transporting  agents  as  rivers  on  the 
land?)  The  work  of  shore  currents  is  discussed  later  (p.  246). 

The  regular  rise  and  fall  of  the  waters  of  the  ocean,  twice 
in  about  twenty-four  hours,  constitute  the  tides.  In  the 
open  ocean  the  tides  are  imperceptible.  Along  the  shores 
the  change  of  level  ranges  from  2  or  3  feet  to  50 
feet  and  more  in  narrow  bays.  For  about  six  hours  the 
water  rises  and  advances  upon  the  shore  (flood  tide),  and 
then  for  an  equal  time  falls  and  recedes  (ebb  tide).  Wide 
flats  are  in  consequence  often  alternately  exposed  to  the 
atmosphere  and  covered  by  the  sea.  In  V-shaped  bays  and 


OCEANS  AND  LAKES 


237 


estuaries,  and  in  narrow  passages  between  islands,  tidal  cur- 
rents may  be  of  great  strength,  and  sometimes  sweep  quan- 
tities of  sediment  back  and  forth  and  erode  the  beds  and 
sides  of  their  channels.  Tides  aid  the  work  of  wind  waves 


., 


FIGS.  253,  254.  —  High  tide  and  low  tide  on  the  coast  of  Maine  at  North 
Haven.  The  rocks  exposed  at  low  tide  but  under  water  at  high  tide  are 
heavily  covered  with  seaweed.  Such  vegetation  often  helps  to  protect 
rocky  coasts  against  wave  erosion.  (Bailey  Willis.) 

by  lifting  and  lowering  them,  and  so  increasing  the  width  of 
their  zone  of  attack  (Figs.  253  and  254). 


THE  SHORES  OF  THE  OCEAN 

The  shores  of  the  ocean  are  zones  of  great  activity.  Here 
is  the  meeting  place  of  land  and  air  and  sea.  The  principal 
coast-line  features  and  offshore  deposits  are  discussed  in 


238  PHYSICAL  GEOLOGY 

the  following  paragraphs.  A  knowledge  of  these  things  aids 
in  determining  the  geographic  changes  of  the  past. 

The  characteristics  of  shore  lines,  and  the  agents  which 
shape  them.  —  The  shores  of  the  northern  continents  are 
characterized  by  great  projections  of  the  land  into  the  sea, 
and  by  great  extensions  of  the  sea  into  the  land.  Large 
irregularities  like  Florida,  Lower  California,  the  Iberian 
Peninsula,  and  Hudson  Bay  are  due  to  diastrophism.  In  a 
late  geological  period  an  upbowing  of  a  part  of  the  marginal 
sea  bottom  made  an  island  of  Florida  which,  by  continued 
movement,  was  attached  to  the  mainland  as  a  peninsula.  A 
geologically  recent  subsidence  let  the  sea  in  over  the  area 
of  Hudson  Bay. 

The  submergence  of  a  coast  land  having  hills  and  valleys 
produces  a  new  shore  line  which  is  irregular  (Fig.  271). 
The  drowned  valley  bottoms  form  bays,  while  the  inter- 
valley  ridges  stand 
forth  as  headlands. 
Isolated  hills  of  the 
old  lowlands  front 
the  new  coast  as 
islands.  Chesa- 

FIG.  255.  —  Diagram  of  a  young    coastal    plain,    peake     Bay,     Dela- 
with  the  old  land  in  the  background.  ware      BaV        and 

many  other  smaller  bays  along  the  eastern  coast  of  the  United 
States  are  drowned  valleys  or  valley  systems.  On  the  other 
hand,  the  emergence  of  a  coastal  strip  tends  to  produce  an 
even,  regular  shore  line,  for  the  edge  of  the  sea  rests  against 
the  gently  sloping  former  bottom  (Fig.  255).  (What  should 
be  the  general  height  of  new  coasts  due  to  (1)  submergence, 
and  (2)  emergence?) 

In  addition  to  the  features  formed  by  diastrophism,  many 
coastal  irregularities  are  due  to  the  work  of  gradational 
agents.  Under  normal  conditions  rivers  erode  but  little  at 
their  mouths,  but  may  build  deltas  into  the  sea  (p.  185). 
Glaciers  descending  into  the  sea  help  to  develop  fiords 


OCEANS  AND  LAKES  230 

(p.  224),  and  may  build  islands  by  depositing  drift.  Loose 
material  is  often  incorporated  in  ice  formed  along  high- 
latitude  coasts  in  winter ;  when  the  ice  breaks  up  in  the 
spring,  this  material  may  be  carried  away  to  be  dropped 
where  the  ice  melts.  Weathering  agents  reduce  sea  cliffs 
and  loosen  material  along  shore,  preparing  it  for  removal 
by  other  agents.  But  most  important  in  shaping  the  details 
of  coast  lines  is  the  work  of  wind  waves  and  of  the  shore 
currents  which  they  generate.  The  features  they  develop 
are  discussed  below. 


EROSION    BY    THE    SEA 

How  the  sea  wears  its  shores.  —  Clear  waves  dashing 
against  cliffs  of  firm,  unjointed  rock  accomplish  little  or  no 
wear.  The  inability  of  waves  to  erode  under  these  circum- 
stances recalls  the  similar  dependence  of  winds,  streams,  and 
glaciers  upon  their  rock  tools.  But  the  conditions  suggested 
rarely  occur.  Usually  the  rocks  of  the  seashore  are  traversed 
by  joints.  If  stratified,  they  contain  bedding  planes.  There 
are  still  other  openings,  and  all  form  weak  places.  With  the 
impact  of  strong  waves,  water  is  forced  into  the  openings 
with  great  pressure.  Furthermore,  the  air  in  the  openings 
is  compressed  by  the  invading  water,  and  then  expands  with 
force  as  the  water  withdraws.  In  these  ways  pieces  of  rock 
are  broken  and  sucked  off,  and  the  openings  enlarged.  Ordi- 
narily, too,  the  water  offshore  is  sufficiently  shallow  for  the 
waves  to  obtain  from  the  bottom  sand,  stones,  and  some- 
times, when  very  strong,  even  large  bowlders,  which  are 
hurled  as  battering-rams  against  the  shore.  Locally,  the  sea 
dissolves  the  rocks  of  its  shore. 

Rate  of  erosion.  —  The  rate  at  which  a  given  coast  is  eroded 
is  determined  by  several  factors.  (1)  Other  things  being 
equal,  strong  waves  obviously  erode  faster  than  weak  ones. 
The  velocity  of  the  winds  which  generated  them,  the  depth 
of  the  water  they  have  traversed,  and  the  distance  they  have 


240  PHYSICAL  GEOLOGY 

come  before  reaching  .the  coast,  all  influence  the  strength  of 
the  waves.  (How  does  each  factor  affect  the  result?)  The 
force  of  waves  has  been  measured  in  connection  with  cer- 
tain engineering  enterprises.  On  the  coast  of  Scotland  and 
among  the  outer  Hebrides,  storm  waves  sometimes  exert  a 
pressure  of  nearly  three  tons  per  square  foot.  (2)  The  rate 


FIG.  256.  —  Wave  erosion  near  Santa  Cruz,  Cal.  The  parallel  channels 
in  the  foreground  are  the  result  of  rapid  wear  along  joint  lines.  (U.S. 
Geol.  Sun.') 

of  wear  is  influenced  by  the  character  and  structure  of  the 
rocks  at  the  shore.  Soft  rocks  wear  faster  than  hard  ones, 
soluble  rocks  faster  than  insoluble  ones,  rocks  with  many 
joints  (Fig.  256)  and  openings  faster  than  rocks  with  few. 
(Other  things  equal,  which  structure  would  occasion  most 
rapid  wear,  (a)  horizontal  beds,  (6)  beds  dipping  abruptly 
toward  the  sea,  (c)  beds  dipping  away  from  the  sea?  Why?) 
(3)  Finally,  the  rate  of  wear  is  influenced  by  the  number  and 
character  of  the  tools  of  the  waves.  The  shallower  the  water 
immediately  offshore,  the  greater  the  number  of  tools  that  are 
likely  to  be  accessible  to  the  waves.  But,  on  the  other  hand, 
if  the  water  be  very  shallow  for  any  considerable  distance 
from  the  shore,  the  velocity  of  the  waves  will  be  so  reduced  by 
friction  with  the  bottom  that,  on  arriving  at  the  shore  line, 
they  will  be  unable  to  erode  effectively.  It  may  be  noted 


OCEANS  AND  LAKES  241 

that  usually  deep  water  fronts  high  coasts,  and  relatively 
shallow  water,  low  coasts. 

On  the  eastern  coast  of  England,  where  the  rocks  are  rela- 
tively weak,  entire  parishes  have  been  washed  away  within  a 
few  centuries ;  in  some  places  the  shore  line  has  retreated  as 
much  as  15  feet  in  a  single  year.  The  south  shore  of  Nan- 
tucket  Island,  Massachusetts,  has  lost  in  places  as  much  as 
6  feet  in  a  year,  and  as  early  as  1835  the  opinion  was  expressed 
that  within  a  few  centuries  the  entire  island  would  be  devoured 
by  the  sea. 

Sea  cliffs  and  terraces.  —  The  chief  topographic  effects  of 
wave  erosion  are  illustrated  by  Figure  257.  The  original  slope 
near  the  water  level  is  indicated  by  the  dotted  line.  Erod- 
ing waves  have  notched  this  slope, 
forming  a  sea  cliff.  The  develop- 
ment and  recession  of  a  sea  cliff 
involve  also  the  formation  and 
widening  at  its  base  of  an  under- 


water platform,  called  the  wave-cut  FlG>  257  _  Diagram  of  sea 
terrace.  Its  surface  represents  the  cliff,  wave-cut  terrace,  and 
lower  limit  of  effective  wave  action.  wave'built  terrace- 
It  slopes  gently  seaward  because,  as  its  width  increases,  the 
strength  of  the  waves  at  its  inner  edge  decreases  (Why?),  and 
they  are  accordingly  able  to  cut  a  less  and  less  distance  below 
sea  level.  (Should  you  expect  the  slope  of  wave-cut  terraces 
to  vary?  If  so,  why?)  At  first  the  material  worn  from  the 
cliffs  is  swept  to  the  edge  of  the  wave-cut  terrace,  and  de- 
posited in  deeper  water.  Here  it  accumulates  to  form  the 
wave-built  terrace,  which  extends  the  wave-cut  terrace  seaward. 
Later,  more  or  less  of  the  waste  of  the  cliffs  remains  here  and 
there  upon  the  terrace  at  their  base  (Why?)  to  form  a  beach. 
Still  later,  when  the  beach  is  developed  more  continuously, 
much  of  the  waste  is  washed  along  it  by  waves  and  shore  cur- 
rents (p.  246).  Wave-formed  terraces  may  become  land  by 
lowering  of  the  sea,  or  by  uplift  of  the  coast  line  (Figs.  258 
and  259). 


242 


PHYSICAL  GEOLOGY 


FIG.  258.  —  Wave-cut  terraces  on  the  California  coast.     (U.S.  Geol.  Sun.) 
How  many  terraces  are  shown  ?    What  is  their  relative  age  ?    Outline 
the  history  of  the  coast  as  recorded  by  the  terraces.     What  changes  are  now 
in  progress  ? 

The  height  of  sea  cliffs  depends  upon  the  elevation  above  sea 
level  of  the  land  at  the  coast.  Their  steepness  varies  with 
(1)  the  strength  and  structure  of  the  rocks,  and  (2)  the 
rapidity  of  wave  cutting  and  of  weathering  upon  the  cliffs 
above.  Loose  material  usually  cannot  stand  in  steep  cliffs. 
Firm  rocks  may  form  vertical  and  even  overhanging  cliffs 


FIG.  259.  —  Raised  beaches,  near  Elie,  Fife.     (Laurie.) 

(Figs.  260  and  261).  (What  rock  structures  favor,  and  what 
ones  oppose,  the  formation  of  steep  cliffs?)  Rapid  cutting  by 
the  waves  tends  to  keep  the  cliffs  steep,  while  the  weathering 
of  the  rocks  of  the  upper  cliffs  and  the  removal  of  the  loosened 
material  tend  to  lessen  their  declivity.  (What  inference 
may  be  made  from  the  fact  that  even  sea  cliffs  containing 


OCEANS  AND  LAKES  243 

rocks  capable  of  standing  in  vertical  faces,  commonly  slope 
sharply  back  toward  the  land?)  The  rapid  weathering  of 
sea  cliffs  is  favored  by  the  absence  of  protecting  talus  (Why 
absent  ?)  and  often  of  vegetation,  and  by  the  frequently  wet 
condition  of  the  rocks  due  to  the  spray.  The  active  issu- 
ance of  ground  water  as  seepage  and  springs  near  the  level  of 
the  sea  often  helps  to  undermine  sea  cliffs. 


FIG.  260.  —  Sea  cliffs  on  the  northern  coast  of  France. 

Sea  caves,  stacks,  natural  bridges.  —  The  enlargement  by 
the  waves  of  a  joint  or  other  opening  in  the  face  of  a  sea  cliff 
may  result  in  a  sea  cave  (Fig.  262),  provided  the  overlying 
rock  is  strong  enough  to  form  a  roof.  Occasionally  a  sea 
cave  is  worn  back  and  up  to  the  surface  of  the  ground  some 
distance  back  from  the  cliff.  Again,  a  fissure  or  joint  may 
form  an  opening  between  the  inner  end  of  a  sea  cave  and  the 
surface  of  the  ground.  Storm  waves  sometimes  drive  spray 
and  water  up  through  such  openings,  which  are  then  called 
blowholes. 

Taking  advantage  of  joint  systems,  waves  sometimes 
quarry  out  the  rocks  about  a  section  of  a  cliff,  leaving  it  as  an 


244  PHYSICAL  GEOLOGY 

island  in  front  of  the  retreating  shore.  From  their  form,  such 
islands  are  frequently  called  stacks  or  chimney  islands  (Fig. 
263). 

Waves  may  cut  through  a  rocky  headland  in  such  manner 
as  to  form  a  natural  bridge  (Fig.  264).     If  the  roof  covering  a 


FIG.  261.  —  Sea  cliffs  in  northwestern  France. 

sea  cave  near  its  mouth  remains  after  the  roof  above  the  inner 
end  of  the  cave  has  collapsed,  a  natural  bridge  also  results. 

While  interesting  because  of  their  picturesqueness,  these 
special  features  of  cliff  shores  have  little  geological  importance. 

The  goal  of  sea  erosion.  —  Just  as  rivers  seek  to  wear  the 
land  to  sea  level,  so  the  waves  of  the  sea,  acting  as  a  horizontal 
saw,  seek  to  cut  the  land  to  a  level  slightly  below  the  surface 


OCEANS  AND  LAKES 


245 


of  the  sea.     Extensive  peneplains  have  been  developed  re- 
peatedly in  the  past,  but,  so  far  as  known,  wave-cut  submarine 


FIG.  262.  —  Sea  caves  on   the  southern  coast   of   California.     (Fairbanks, 

U.S.  Geol.  Surv.) 

When  the  upper  cave  was  cut  it  stood  in  the  same  relation  to  sea  level 
that  the  lower  one  now  does.  Since  it  was  formed  th£  land  has  therefore 
been  elevated  with  reference  to  the  level  of  the  ocean. 

plains  of  great  extent  have  not  been  formed.  This  is  because, 
as  already  indicated,  waves  drag  bottom  across  the  sub- 
marine flat  which  they  cut,  and  so  become  weaker  as  the  flat 
becomes  wider.  The 
gradual  subsidence  of  a 
coast  and  marginal  sea 
bottom  aids  in  the  ex- 
tension of  a  wave-cut 
plain  by  gradual^  in- 
creasing the  depth  of  the 
water  upon  it,  and  so 
maintaining  the  vigor  of 
the  waves  at  the  shore. 
Gradual  emergence,  on 
the  other  hand,  opposes  „ 

'  FIG.  263.  —  Stacks  on  the  west  coast 

the  formation  of  an  exten-  of  France. 

B,  <V  B.  GEOL. 14 


246 


PHYSICAL  GEOLOGY 


sive  wave-cut  plain.  Plains  of  marine  denudation,  like  base- 
level  plains,  cut  indifferently  across  beds  of  varying  structure 
and  hardness. 


FIG.  264.  —  Natural  bridge  on  the  coast  of  Califor- 
nia, near  Santa  Cruz. 


TRANSPORTATION   AND   DEPOSITION   ALONG   THE   SEASHORE 

The  beach  and  transportation.  —  When  waves  come  in  to 
the  shore  obliquely,  some  of  the  bottom  material  is  moved 
up  and  at  the  same  time  along  the  beach.  Carried  out  by  the 

undertow,  it  is 
again  swept  up 
and  along  the 
beach, /and  by  a 
continuation  of 
the  process  trav- 
els alongshore  by 
a  series  of  zigzag 
paths.  Further- 
more, winds  blow- 
ing obliquely  upon  a  beach  generate  a  current  which  moves 
alongshore,  and  is  known  as  the  shore  current  or  littoral  cur- 
rent. Littoral  currents  are  not  strong,  but  when  sand  particles 
and  pebbles  are  lifted  wholly  or  partially  by  the  waves  or  the 
undertow,  the  current  is  able  to  move  them  a  slight  distance 
along  the  beach  in  the  direction  of  its  movement. 

The  beach  is  the  roadway  along  which  the  shore  drift  is 
transported.  (What  determines  its  width?)  The  material 
of  unprotected  beaches  at  the  foot  of  sea  cliffs  is  coarse,  for 
the  water  is  agitated  vigorously,  and  only  relatively  heavy 
material  can  remain ;  silt  is  carried  seaward  by  the  undertow, 
or  along  the  coast  by  shore  currents  to  more  sheltered  places. 
In  sheltered  bays,  and  along  low,  protected  shores,  the  beach 
material  is  likely  to  be  fine  sand  or  mud. 

As  the  material  of  a  beach  is  moved,  sometimes  to  a  depth 
of  several  feet  on  exposed  coasts  during  severe  storms,  the 
particles  are  worn  and  crushed,  and  may  be  reduced  at  last 


OCEANS  AND  LAKES 


247 


to  fine  mud.  The  final  reduction  of  beach  material  is  accom- 
plished with  extreme  slowness,  however,  for  the  particles  are 
becoming  smaller  and  therefore  lighter,  and  each  is  surrounded 
by  a  film  of  water,  which  acts  as  a  cushion.  All  blows,  there- 
fore, come  to  be  weak  blows.  Furthermore,  before  it  is  re- 
duced completely,  the  material  is  apt  to  be  removed  from  the 
mill  of  the  beach,  and  deposited  in  quieter  water.  It  is  re- 
placed by4iew  material  worn  from  the  cliffs  by  the  waves, 
or  brought  from  the  land  by  streams. 


FIG.  265.  —  Hooked  spit  at  entrance  to  Smithtown  Harbor,  Long  Island. 

(Buffet.) 

Features  formed  by  deposition  of  shore  drift.  —  When 
shore  currents  reach  the  entrance  of  a  harbor,  or  some  other 
abrupt  bend  in  the  shore  line,  they  commonly  continue  in  the 
direction  in  which  they  had  been  moving,  instead  of  turning 
with  the  coast.  They  accordingly  pass  from  the  shallow 
water  of  the  outer  beach  into  deeper  water,  where  they  drop 
their  load.  The  result  is  an  embankment,  known  as  a  spit. 
Waves  may  build  spits  above  sea  level,  and  the  winds  may 
then  form  dunes  upon  them.  Many  spits  accordingly  pre- 
sent irregular  surfaces,  and  support  hills  which  rise  10  to  40 
or  more  feet  above  sea  level.  Spits  are  tied  at  one  end  to  the 
beach,  of  which,  indeed,  they  form  an  extension,  and  are 
bounded  by  deep  water  at  the  free  end.  The  ends  of  spits  are 
frequently  bent  by  storm  waves.  Bent  spits  are  called 
hooks  (Fig.  265).  Occasionally  the  hook  at  the  end  is  closed 


248 


PHYSICAL  GEOLOGY 


FIG.  266.  —  A  bar.     Sea  cliffs  in  distance. 

completely,  forming  a  loop.  Many  spits  have  been  built  en- 
tirely across  the  mouths  of  harbors,  and  joined  to  the  beach 
beyond.  Such  completed  spits  are  bars  (Fig.  266).  Bars 
sometimes  connect  islands  with  the  mainland,  thus  making 
land-tied  islands.  Plate  XIV  shows  spits,  hooks,  bars,  land- 
tied  islands,  etc.,  on  the  coast  of  Long  Island. 

Barrier  islands.  —  Storm  waves  drag  on  shelving  bottoms 
at  some  distance  from  the  shore.  They  drop  most  of  their 
load  where  they  break,  along  a  line  roughly  parallel  with  the 


FIG.  267.  —  Diagram  of  barrier  islands  and  lagoon. 


FIG.  268.  —  Diagram  showing  a  later  stage  in  the  development  of  the  coast 
represented  by  Figure  267.  Dunes  have  formed  on  the  barrier  islands. 
Marshes  cover  much  of  the  area  of  the  lagoon. 


249 


250 


PHYSICAL  GEOLOGY 


shore.  By  this  means,  and  also  by  the  addition  of  material 
washed  outward  by  the  undertow,  a  ridge  may  be  built  above 
the  sea  surface,  forming  a  long,  narrow  sand  island  called  a 
barrier  (Fig.  267).  Dunes  may  presently  be  built  upon  its 
surface,  and  vegetation  may  obtain  a  foothold.  Barrier 
islands  extend  along  much  of  the  coast  of  the  United  States 
from  New  Jersey  to  Texas.  They  inclose  shallow-water  areas 

called  lagoons,  which  are  being 
filled  gradually  by  wash  from 
the  mainland  and  the  islands, 
by  migrating  sand  dunes  and 
wind-borne  dust,  and  by  en- 
croaching vegetation  (Figs. 
268  and  269).  Thus  lagoons 
may  become  marshes  and 
finally  be  added  to  the  main- 
land area.  If,  on  the  other 
hand,  a  barrier  comes  to  re- 
ceive less  sand  from  the  bot- 
tom or  from  other  places 
alongshore  than  is  carried 
away  by  waves  and  currents, 
it  may  be  destroyed,  together 
with  the  lagoon  marsh  it  in- 
closed. Occasional  breaks  (in- 
lets) in  barriers  are  kept  im- 
perfectly open  by  tidal  scour, 
by  the  outflow  of  waters  from 
the  mainland,  or  by  both. 
The  tides  of  the  Gulf  of  Mexico  being  weak,  the  barrier 
islands  off  the  coastal  plain  of  Texas  have  few  breaks  (Fig. 
274),  a  fact  which  has  retarded  the  commercial  development 
of  the  region.  Most  lagoons  are  accessible  only  to  boats  of 
light  draft,  and  are  not  frequented  by  extensive  commerce. 

Influence  of  plants  and  animals  upon  shore  lines.  —  Plants 
affect  shore  lines  in  two  important  ways ;  they  often  protect 


'A        I 


FIG.  269.  —  A  portion  of  the  New 
Jersey  coast. 


OCEANS  AND  LAKES  251 

the  shore  against  wave  erosion,  and  certain  plants  which  live 
in  shallow  salt  water  aid  effectively  in  the  extension  of  the 
land  seaward. 

Along  a  rocky  shore  one  may  often  see  at  low  tide  that  the 
rocks  are  covered  with  a  mat  of  seaweed  and  other  vegetation 
which,  during  storms,  acts  as  a  buffer  to  deaden  the  force  of  the 
waves  (Fig.  254). 


FIG.  270.  —  Mangroves  on  shore  of  Biscayne  Bay,  near  Lemon  City,  Florida. 
(R.  M.  Harper.) 

The  mangrove  flourishes  on  shallow,  muddy  bottoms  off 
many  low-latitude  coasts  not  exposed  to  strong  surf.  Florida 
furnishes  good  examples  (Fig.  270).  The  many  widely  spread- 
ing roots  start  from  above  the  surface  of  the  water  and  even 
from  the  lower  limbs,  forming  a  tangle  which  serves  to  catch 
and  hold  the  sediment  washed  from  the  land.  The  effect  of 
great  numbers  of  trees  is  to  occasion  the  lodgment  of  large 
quantities  of  sediment.  Certain  low,  marshy  coastal  plains 
appear  to  have  originated  in  this  way.  A  similar  work  is  done 
by  grasses  which  grow  in  coastal  lagoons  and  marshes. 

STAGES   IN   SHORE-LINE   DEVELOPMENT 

Shore  lines  tend  to  pass  through  regular  cycles  of  develop- 
ment. A  coastal  cycle  is  begun  by  diastrophism,  emergence 


252 


PHYSICAL  GEOLOGY 


FIG.  271.  —  Diagram  of  a  coast  that  has  been 
submerged  recently. 


tending  to  produce  an  even,  regular  coast,  submergence  an 
irregular  one  (p.  238).     In  either  case,  waves  and  currents 

tend  first  to  in- 
crease the  irregular- 
ity of  the  shore  line, 
and  finally  to  make 
it  smooth.  Unequal 
wave  erosion  and 
the  building  of  spits, 
hooks,  barriers,  etc., 
increase  the  length 
of  a  shore  line. 
Later,  the  wearing 
back  of  the  head- 
lands and  the  filling 
up  of  the  indenta- 
tions, the  complet- 
ing of  bars  and  the 
filling  in  of  the  bays 
and  lagoons  they  in- 
traighten  and 


FIG.  272.  —  Diagram  showing  the  coast  repre- 
sented by  Figure  271  after  it  has  been  modified 
by  streams  and  shore  agents.  Marshy  bay-head 
deltas  have  been  formed  by  the  larger  rivers. 


Shore  currents  and  waves  have  built  hooks  and 
bars.  The  islands  are  partly  consumed.  Sub- 
marine terraces  front  the  cliffs.  The  material 
worn  from  the  land  is  spread  over  the  ocean- 
bottom  as  sheets  of  sand  and  mud. 


Waves  have  cut   back  the   headlands  in   cliffs.    Simplify    the    COast. 

Figures  271,  272, 
and  273  show  suc- 
cessive stages  in  the 
development  of  an 
embayed  (drowned) 
coast.  Figure  274 
shows  shore  deposits 
shutting  in  bays, 
and  tending  to  sim- 
plify the  coast  line ; 
thus  far,  however, 

FIG.  273.— A  still  later  stage  in  the  development  they  have  increased 

of  the  coast  shown  in  the  two  preceding  Figures.  the  snOre-line  mile- 
Bay  filling  and  cliff  recession  have  produced  a 

nearly  straight  shore  line.  age. 


OCEANS  AND  LAKES 


253 


There  are  several  points  of  similarity  between  coastal  cycles 
and  the  erosion  cycles  of  rivers.  First  a  river  system  roughens 
the  surface  of  its  basin,  increasing  its  relief ;  finally  it  reduces 
it  to  a  smooth  plain,  near  sea  level.  As  indicated  above, 
waves  and  currents  normally  increase  to  a  maximum  the  irreg- 
ularities of  a  coast,  and  finally  reduce  them  to  a  minimum. 
An  essential  difference  is 
that  the  irregularities  of 
the  river  basin  are  verti- 
cal irregularities,  while 
those  of  the  shore  line  are 
horizontal.  In  each  case 
the  cycle  of  development 
is  introduced  by  diastro- 
phism.  Diastrophism  in 
each  case  frequently  ter- 
minates incompleted 
cycles. 

From  what  has  been 
said  already,  it  will  be 
evident  that  the  rate  at 
which  a  coast  develops  de- 
pends on  (1)  the  strength 
of  the  waves  and  currents, 
and  (2)  the  resistance  of 
the  rocks  of  the  coast.  The  outer  coast  of  the  end  of  Cape 
Cod  is  farther  advanced  than  the  inner,  though  the  material 
is  the  same,  because  the  outer  side  is  exposed  to  the  vigorous 
waves  and  currents  of  the  open  sea,  while  the  inner  side  is 
somewhat  protected  (Fig.  275).  The  coast  of  Maine  is  in 
general  in  a  youthful  stage  of  development,  though  its  exposure 
is  comparable  to  that  of  the  outer  coast  of  Cape  Cod,  and 
waves  and  currents  have  worked  upon  it  for  nearly  as  long. 
The  explanation  lies  in  the  fact  that  the  material  of  the  Cape 
Cod  coast  is  loose  glacial  drift,  while  most  of  that  of  the 
Maine  coast  is  resistant  bedrock. 


FIG.  274.  —  Portion  of  the  Texas  coast, 
showing  barrier  islands  and  the  tend- 
ency of  shore  deposition  to  simplify  the 
coast  line. 


254 


PHYSICAL  GEOLOGY 


CHANGES  IN  THE  RELATION  OF  LAND  AND  SEA 

Sea  level  is  geologically  a  critical  level.  It  is  the  level  to 
which  the  agents  of  degradation  seek  to  reduce  the  land. 
The  activities  which  are  dominant  on  the  land  above  it 
are  different  from  those  dominant  in  the  sea  below  it. 
Great  and  repeated  incursions  of  the  sea  upon  the  land, 


FIG.  275.  —  End  of  Cape  Cod. 

alternating  with  great  extensions  of  the  land  at  the  expense 
of  the  sea,  have  been  among  the  most  important  events  in 
the  past  history  of  the  continents,  for  they  have  changed  the 
dominant  processes  over  vast  areas,  have  modified  climates,  and 
have  changed  the  distribution  and  conditions  of  plant  and 
animal  life. 


OCEANS  AND  LAKES  255 

The  causes  of  changes  in  sea  level  are  several  in  number,  aside 
from  those  changes  due  to  waves,  tides,  etc.,  the  geological 
effects  of  which  have  already  been  considered.  Sediment  from 
the  land  builds  up  the  ocean  floor,  and  so  raises  the  surface  of 
the  sea.  Submarine  volcanic  accumulations  and  the  deposits 
of  corals  and  shell-bearing  sea  life  have  the  same  effect.  The 
elevation  of  a  portion  of  the  ocean  bed  would  also  raise  the  sea 
surface,  while  the  down  warping  of  a  part  of  the  sea  floor 
would  have  an  opposite  effect.  The  lowering  of  a  coastal  area 
below  the  sea  would,  provided  there  were  no  movement  of  the 
ocean  bottom  to  offset  it,  increase  the  area  of  the  ocean,  but 
lower  the  level  of  its  surface.  An  increase  or  decrease  in  the 
aggregate  amount  of  land  water  and  ice  would  lower  or  raise 
the  surface  of  the  ocean.  It  has  been  estimated  that  if  all 
the  snow  and  ice  on  all  the  lands  were  melted  and  returned 
to  the  sea,  it  would  raise  the  sea  surface  by  some  30  feet. 
Since  the  oceans  are  all  connected  with  one  another,  each  of 
the  above  changes  would  affect  the  ocean  surface  everywhere 
and  by  an  equal  amount.  In  various  other  ways  the  surface  of 
the  sea  is  affected  unequally.  For  example,  coastal  mountains 
attract  the  ocean  waters,  so  that  the  neighboring  sea  surface 
is  higher  than  that  at  a  distance.  Any  notable  change  in  the 
mass  of  a  land  area  would  accordingly  affect  the  sea  level.  The 
above  considerations  help  to  explain  the  numerous  changes 
in  the  distribution  of  land  and  sea  that  are  discussed  in  the 
historical  Chapters. 

OCEAN  DEPOSITS 

LAND-DERIVED   DEPOSITS 

The  great  bulk  of  the  land-derived  deposits  in  the  ocean 
was  brought  to  the  sea  by  rivers.  The  sea  wears  from  its 
shores  perhaps  one  thirtieth  or  one  fortieth  the  quantity  of 
material  furnished  by  streams.  Contributions  which  must  be- 
come very  important  in  the  course  of  ages  are  also  made  by 
winds  and  by  glaciers. 


256  PHYSICAL  GEOLOGY 

Distribution.  —  Fine  dust  from  the  land  is  carried  by  winds 
to  all  parts  of  the  ocean.  The  deposition  in  the  ocean  of 
land-derived  sediments  is  accordingly  as  widespread  as  the 
sea  itself.  This  has  undoubtedly  been  true  ever  since  the 
oceans  were  formed.  But  most  of  the  waste  from  the  land 
settles  within  two  or  three  hundred  miles  of  the  shore.  Cer- 
tain powerful  river  currents  carry  material  much  farther  out 
to  sea.  The  Kongo  River,  for  example,  is  said  to  project  its 
current  600  miles  from  shore,  and  the  Ganges  River  nearly 
1000  miles.  Such  cases  are,  however,  very  exceptional. 
The  marginal  sea  bottoms  are  the  great  areas  of  sedimentation. 
The  distribution  of  the  important  land-derived  (terrigenous) 
deposits  is  shown  by  Figure  276. 

Continental  shelves.  —  Parts  of  the  continental  shelves 
(p.  233)  may  be  to  some  considerable  degree  a  product  of  the 
long-continued  offshore  accumulation  of  the  waste  of  the  land. 
Obviously,  however,  a  continental  shelf  might  be  formed  by 
the  submergence  of  a  coastal  plain,  due  either  to  its  depression 
or  to  an  elevation  of  the  surface  of  the  sea.  Soundings  along 
the  eastern  coast  of  the  United  States  and  elsewhere  have 
shown  that  valley  depressions  extend  across  the  continental 
shelf  from  the  mouths  of  various  rivers.  Since  these  valleys 
must  have  been  cut  above  sea  level,  the  sections  of  the  con- 
tinental shelf  in  which  they  occur  appear  to  be  due  to  the 
submergence  of  former  land  areas.  Continental  shelves  may 
also  be  in  part  plains  of  marine  denudation. 

Character  of  land-derived  deposits.  —  The  more  important 
points  concerning  both  the  character  and  the  structure  of  de- 
posits of  land-derived  sediments  were  noted  in  the  discussion 
of  sedimentary  rocks  (pp.  35-37).  It  will  be  remembered  that 
the  gravel,  sand,  and  mud  are  shifted  about  and  worked  over 
by  waves  and  currents,  often  for  long  periods  before  reaching 
a  final  resting  place.  In  the  process  the  materials  are  sorted, 
and  beds  of  each  kind  result,  which  grade  into  one  another 
both  vertically  and  horizontally.  Most  of  the  gravel  comes 
to  rest  close  to  shore  in  depths  of  50  feet  or  less,  but  fine  sand, 


(257^ 


258  PHYSICAL  GEOLOGY 

and  even  mud,  find  permanent  lodgment  in  sheltered  bays,  and 
off  low  coasts  where  wave  action  is  weak.  On  the  other  hand, 
coarse  material  may  extend  far  out  from  shore,  provided  the 
water  is  shallow  and  the  waves  are  strong.  Most  of  the  sand 
is  deposited  beyond  the  gravel,  while  beyond  the  sand  fine 
muds  are  spread  over  the  ocean  floor  to  the  limit  of  sediments 
brought  by  streams  from  the  land. 

Blue  mud  is  the  most  extensive  land-derived  deposit,  and 
is  estimated  to  cover  over  14,000,000  square  miles  of  the  sea 
bottom  (nearly  five  times  the  area  of  the  United  States).  It 
commonly  contains  tiny  particles  of  numerous  minerals,  and 
is  blue  because  the  organic  matter  present  prevents  the  oxida- 
tion of  the  iron.  Red  mud  occurs  over  relatively  small  areas; 
here  the  clay  contained  so  much  Fe203  when  brought  to  the 
sea  that  it  has  been  only  partially  deoxidized.  Green  mud 
and  greensand  "occur  over  an  area  equal  to  about  one  third 
that  of  the  United  States,  and  owe  their  color  to  the  relatively 
large  amount  of  the  mineral  glauconite  present.  Glauconite 
is  a  complex  mineral  containing  alumina,  potash,  and  iron. 
Extensive  deposits  of  greensand  now  form  part  of  the  coastal 
plain  of  New  Jersey. 

ORGANIC   DEPOSITS   AND   RED   CLAY 

Organic  deposits  are  composed  not  of  materials  derived 
directly  from  the  land,  but  principally  of  materials  that  were 
brought  in  solution  by  rivers  to  the  sea,  taken  from 
solution  in  the  sea  water  by  plants  or  animals  and  built  into 
their  shells  or  other  hard  parts,  and  deposited  upon  the  ocean 
bottom  at  the  death  of  these  organisms. 

Oozes.  —  This  term  is  applied  to  fine  oceanic  muds  of  or- 
ganic origin.  The  various  oozes  are  named  from  the  organisms 
whose  remains  contributed  most  to  the  deposit.  Often,  how- 
ever, the  leading  constituent  of  an  ooze  makes  up  only  20 
or  30  per  cent  of  the  total  deposit.  Globigerina  ooze 
(Fig.  277)  is  a  calcareous  deposit  which  takes  its  name  from  a 


OCEANS  AND  LAKES 


259 


FIG.    277.  —  Globigerina     ooze.        Magnified 
about  20  diameters.     (Murray  and  Renard.) 


genus  of  Foraminifera,   microscopic   animals  of   extremely 

simple  structure.     Oozes  of  this  class  cover  nearly  50,000,000 

square  miles  of  the 

ocean    bed.      The 

largest  area  is  in  the 

Atlantic  'Ocean  (Fig. 

276).     Radiolarian 

ooze  is  also  made  up 

of  the  remains  of  a 

group   of   tiny,   one- 

"if 


celled  animals,  but  is 
composed  of  silica 
instead  of  calcium 
carbonate.  It  is  con- 
fined, so  far  as  known,  to  the  Pacific  and  Indian  oceans  (Fig. 
276).  Diatom  ooze  is  a  siliceous  deposit  composed  of  the 
cases  of  minute  plants  known  as  diatoms.  The  largest  de- 
posit is  in  the  Antarctic  Ocean  (Fig.  276).  Various  other 
deposits  of  similar  origin  receive  special  names. 

Coral  deposits.  —  Reef-building  corals  are  limited  to  ocean 
waters  whose  temperature  does  not  fall  below  68°  Fahrenheit. 
They  are  restricted  also  to  places  where  the  water  is  clear  and 
not  over  100  feet  deep.  This  confines  them  to  the  shallow 
areas  of  tropical  seas,  and  prevents  their  growth  off  the 
mouths  of  large  rivers  where  the  waters  are  muddy.  They 
thrive  best  where  the  water  is  agitated  vigorously  by  waves 
and  currents.  This  insures  a  continual  supply  of  food,  oxygen, 
and  calcium  carbonate,  and  removes  the  carbon  dioxide. 
Reef-building  corals  live  in  colonies,  many  of  which  look  like 
stubby  plants.  Each  polyp  consists  of  a  fleshy,  cylindrical 
sac  with  an  opening  at  the^top  which  serves  as  a  mouth,  and  is 
surrounded  with  armlike  feelers.  Each  polyp  absorbs  cal- 
cium carbonate  from  the  sea  water,  and  builds  it  into  the 
stony  framework  which  supports  the  colony. 
;  Coral  reefs  are  of  several  classes.  Those  extending  along 
the  shore  and  attached  to  it  are  fringing  reefs.  Those  which 


260  PHYSICAL  GEOLOGY 

are  separated  from  the  shore  by  a  channel  or  lagoon  are 
barrier  reefs.  Rudely  circular  reefs  inclosing  a  central  la- 
goon are  atolls. 

Coral  limestone  is  also  of  several  kinds.  As  the  reefs  are 
built  up  toward  the  surface  of  the  sea,  they  are  eroded  by  the 
waves.  The  larger  wave-worn  fragments  gather  near  the  reef, 
and  may  be  cemented  into  firm  rock  by  the  deposition  of  cal- 
cium carbonate  from  the  sea  water.  Fine  coral  mud,  ground 
up  by  storm  waves,  is  deposited  over  wide  areas  at  a  distance 
from  the  reef,  and  when  solidified  forms  a  dense,  fine-grained 
limestone.  On  coral  beaches  the  sea  water  sometimes  de- 
posits concentric  layers  of  calcium  carbonate  around  particles 
of  sand.  A  rock  composed  of  tiny  spheres  that  have  been 
built  up  in  this  way  is  called  oolite.  Oolitic  texture  is  pro- 
duced also  in  other  ways.  Figure  276  shows  the  general 
distribution  of  coral  muds  and  sands. 

Since  their  appearance  in  an  early  geological  period,  corals 
have  been  important  rock  makers.  Ancient  reefs  with  all  the 
characteristics  of  modern  ones  occur,  for  example,  near  Mil- 
waukee, Wisconsin,  and  at  Louisville,  Kentucky,  where  they 
occasion  the  rapids  in  the  Ohio  River.  The  geography  of 
these  places  at  the  time  the  corals  lived  may  be  inferred  from 
the  conditions  which  govern  the  present  distribution  of  reef- 
building  corals. 

Red  clay.  —  The  most  extensive  ocean  deposit,  covering  over 
51,000,000  square  miles,  is  known  as  red  clay.  It  is  character- 
istic of  the  deeper  sea,  remote  from  land  (Fig.  276).  Red 
clay  consists  in  part  of  the  insoluble  residue  of  shells,  and  in 
part  of  the  products  of  the  alteration  and  decay  of  volcanic 
ash,  dust,  and  pumice,  which  floats  until  it  becomes  water- 
logged, and  is  drifted  great  distances  by  ocean  currents. 
Fine  dust  from  the  land  and  from  the  combustion  of  meteor- 
ites during  their  passage  through  the  air  also  contributes 
with  extreme  slowness  to  the  deposit. 

It  is  important  to  note  that  few,  if  any,  of  the  rock  systems 
of  the  land  correspond  to  the  deposits  that  are  now  making 


OCEANS  AND  LAKES 


261 


in  the  deeper  parts  of  the  ocean,  and  that  they  do  correspond 
to  the  sediments  gathering  on  the  continental  shelves  and  in 
the  relatively  shallow  seas. 

LAKES 

Distribution  and  origin.  —  Lakes  range  in  size  from  tiny 
ponds  a  few  feet  across  to  Lake  Superior,  between  31,000  and 
32,000  square  miles  in  extent.  They  vary  in  depth  from 


0         I         2        3  M 

FIG.  278.  —  Lakes  near  Pontiac,  Michigan.     Illustrates  the  abundance  of 
lakes  in  parts  of  the  glaciated  region. 

inches  to  5618  feet  in  the  deepest  part  of  Lake  Baikal,  in 
southern  Siberia.  Most  lakes,  however,  are  rather  shallow. 
While  they  occur  at  all  altitudes  from  below  sea  level  to 
thousands  of  feet  above  it  (the  surface  of  Lake  Titicaca  is  at 


262  PHYSICAL  GEOLOGY 

12,500  feet)  and  in  practically  every  latitude,  they  are,  never- 
theless, distributed  very  unevenly  over  the  surface  of  the  land. 
Lakes  abound  in  the  recently  glaciated  areas  of  northeastern 
United  States,  Canada,  and  Europe.  Nearly  one  third  of 
the  surface  of  Finland  is  covered  with  lakes  and  marshes. 
Maine  has  1620  lakes,  and  it  is  said  that  Minnesota  may 
have  8000.  Figure  278  illustrates  their  abundance  in  parts 
of  Michigan.  In  these  regions,  as  already  indicated  (p.  214), 


FIG.  279.  —  Lake   in   an   ice-scoured   rock   basin.     Northern   Washington. 
(Russell,  U.S.  Geol.  Surv.) 

they  commonly  occupy  (1)  ice-scoured  rock  basins,  (2)  hol- 
lows in  the  unevenly  deposited  drift  (Fig.  225),  or  (3)  the  un- 
filled depressions  of  drift-choked  preglacial  valleys.  Lakes  are 
numerous  also  in  northwestern  United  States,  western  Canada, 
the  Alps,  and  elsewhere,  in  glaciated  mountain  valleys.  Most 
of  them  fill  depressions  (1)  behind  morainic  dams  (Figs.  238 
and  240),  or  (2)  gouged  out  by  the  glaciers  which  formerly 
occupied  the  valleys  (Figs.  239  and  279).  Lakes  are  common 


OCEANS  AND  LAKES 


263 


along  many  ocean  shores,  back  of  bars  and  barriers  (Fig.  280) . 
They  occur  in  irregular  belts  or  groups  in  certain  interior 

basins     and    plateaus,     

as  in  central  Asia  and 
parts  of  Africa.  Some 
of  the  larger  lakes  of 
these  areas  are  in  basins 
formed  by  the  down 
warping  or  down  fault- 
ing of  portions  of  the 
surface  (Fig.  281);  some 
have  formed  behind 
dams  of  stream-swept 
waste ;  some  have  gath- 
ered in  basins  of  other 
origin.  Lakes  are  common  features  of  many  broad  flood  plains, 
representing  abandoned  sections  of  shifting  rivers  (p.  180). 
They  are  characteristic,  too,  of  many  large  deltas  (p.  186). 
A  few  lakes  occupy  basins  that  have  originated  in  still  other 
ways.  Avalanches  and  lava  flows  may  dam  rivers.  The 
surfaces  of  recently  emerged  coastal  plains  may  contain  shal- 


1  Miles 


FIG.  280.  —  Coastal  lakes  shut  off  from  the 
sea  by  sand  bars.  South  coast  of  Long 
Island. 


f  f     f  f 

FIG.  281.  —  Section  across  the  Dead  Sea. 


//,  faults. 


low  depressions.  Certain  areas  underlain  by  limestone  have 
many  sink  holes  (p.  118).  These  and  other  depressions  may 
contain  lakes. 

The  general  conditions  necessary  for  the  formation  of  a  per- 
manent lake  will  be  inferred  readily  from  what  has  already 
been  said.  They  are  (1)  a  depression  below  the  surface  of  the 
surrounding  country,  (2)  whose  bottom  is  beneath  the  lowest 
level  of  the  water  table. 


264  PHYSICAL  GEOLOGY 

Functions  of  lakes.  —  The  geological  functions  of  lakes  are 
several  in  number.  (1)  They  tend  to  increase  the  rainfall  and 
to  equalize  the  temperature  of  their  surroundings,  making  the 
summers  cooler  and  the  winters  warmer  than  they  would  be 
otherwise.  Since  the  character  of  the  climate  affects  various 
geological  processes,  this  tendency  is  not  unimportant.  (2) 
Mechanical  and  chemical  deposits,  discussed  further  below, 
are  being  made  in  lakes.  Although  but  a  few  lakes  are  of 
great  area  (some  ten  only  exceed  10,000  square  miles  in  extent), 
yet  the  aggregate  area  of  lake  sedimentation  is  very  large. 
(3)  Lakes  filter  the  waters  of  their  tributary  streams  and  regu- 
late the  volume  of  outflowing  streams,  preventing,  or  tending 
to  prevent,  destructive  floods.  Thus  they  influence  erosion 
throughout  the  areas  affected  by  the  streams  which  flow  from 
them.  (4)  Many  kinds  of  plants  and  animals  dwell  in  lakes, 
and  the  bodies  of  land-inhabiting  animals  are  often  washed  in  by 
streams.  So  far  as  these  are  capable  of  preservation,  they  may 
be  converted  into  fossils  in  the  growing  lake  sediments.  Thus 
the  lake  deposits  of  former  ages  often  afford  a  valuable  record 
of  the  lake  and  land  life  of  the  times  when  the  lakes  existed. 

Processes  in  operation  in  lakes.  —  The  changes  in  progress 
in  lakes  correspond  closely  to  those  taking  place  in  the  ocean, 
and  discussed  in  preceding  pages.  (1)  Winds  generate  waves 
more  easily  in  fresh  than  in  salt  water,  because  the  former  is 
lighter,  and  when  lakes  are  sufficiently  large  and  deep,  strong 
storm  waves  develop.  Where  they  wear  the  shore,  cliffs 
and  terraces  are  formed  (Figs.  282  and  283).  The  material 
worn  from  the  cliffs  is  swept  by  the  undertow  into  deeper 
water,  or  transported  alongshore  by  waves  and  wind-driven 
currents  and  built  into  beaches,  spits,  bars,  etc.  In  the  pro- 
cess the  shore  drift  is  assorted  and  worn.  The  general  effect  of 
the  work  of  waves  and  currents  is  to  increase  the  area  of 
the  lakes,  but  at  the  same  time  to  make  them  shallower. 
(2)  Streams  and  rains  wash  material  into  lakes  from  the  tribu- 
tary slopes.  In  many  cases  the  sediment  brought  in  by  rivers 
accumulates  at  their  mouths  to  form  deltas.  Lake  deltas  built 


OCEANS  AND  LAKES 


265 


by  mountain  torrents  are  likely  to  be  composed  of  coarse  ma- 
terial and  to  have  very  steep  fronts.     (3)  The  slowly  decaying 


FIG.  282.  —  An  undercut  cliff  on  the  shore  of  Kelleys  Island,  Lake  Erie. 
(H.  E.  Wilson.) 

remains  of  plants  accumulate  upon  the   bottoms  of  lakes, 

especially  about  their  shallow  borders,  and  tend  to  shoal  them. 

(4)  Various  kinds  of  shell-building  animals  inhabit  the  waters 

of  most  lakes,  and  at  death 

their  shells  help  to  fill  the 

lake  basins.      (5)  Winds 

blow  some  fine  material 

into  all  lakes  and  much 

into  many.     (6)  Certain 

minerals    are     deposited 

from  the  waters  of  many 


FIG.  283.  — Sketch  of  the  Bowsprit,  Point 
aux  Barques,  Michigan.  The  island  was 
formerly  a  part  of  the  mainland,  from 
which  it  was  isolated  by  wave  erosion. 
The  waves  are  undercutting  the  island, 
and  will  destroy  it  in  time. 


lakes,  especially   in   arid 

regions.     (7)  Most  lakes 

in  humid  regions  fill  their 

basins  above  the  level  of 

the  lowest  point  in  the  rim,  and  so  have  outflowing  streams. 

Such  streams  wear  the  outlets  lower,  rapidly  when  they  are  of 

large  volume  and  the  rock  is  soft,  but  very  slowly  when  the 

B.  &  B.  GEOL. 15 


266  PHYSICAL  GEOLOGY 

rock  is  resistant,  because,  as  noted  above,  lakes  act  as  settling 
basins  for  the  sediment  brought  in  by  their  tributaries,  so  that 
the  issuing  streams  have  at  the  outset  few  tools.  Still  other 
less  important  changes  are  taking  place  in  certain  lakes. 

The  fate  of  lakes.  —  It  is  apparent  from  the  foregoing  dis- 
cussion that  lakes  are  short-lived  features,  and  accordingly 
that  all  existing  lakes  are  of  geologically  recent  origin.  Since 
deposits  of  every  sort  displace  an  equal  volume  of  W;%ter,  it  is 
evident  that  if  continued  long  enough)  deposition  will  com- 
pletely fill  a  lake  basin  and  obliterate  the  lake*  Vegetation  is 
often  a  chief  factor  in  the  last  stages  of  lake  filling  (Fig.  284). 


FIG.  284.  —  A   pond   nearly   destroyed    by    encroaching    vegetation.     Yel- 
lowstone Park.     (Fairbanks.) 

Furthermore,  the  outflowing  stream  may  cut  the  outlet  of  a 
lake  below  the  level  of  the  bottom  of  its  basin,  and  so  drain  off 
all  the  water.  Most  lakes  are  being  destroyed  slowly  in  both 
ways.  It  has  been  estimated  that  although  Minnesota  now 
has  perhaps  8000  lakes,  large  and  small,  it  will  contain  in  fifty 
years  fewer  than  5000.  Many  of  its  lakes  are  really  ponds, 
well-nigh  effaced.  This,  of  course,  does  not  mean  that  the 
earth  will  presently  be  without  lakes.  Existing  lakes  will 
be  succeeded  by  others,  just  as  they  have  been  preceded  by 
many  generations  of  earlier  ones. 


OCEANS  AND  LAKES  267 

Extinct  lakes.  —  Beds  deposited  in  lakes  now  extinct  cover 
extensive  areas  in  various  regions.  Their  origin  is  indicated 
by  some  or  all  of  the  following  characteristics:  (1)  In  many 
cases  the  beds  show  a  concentric  arrangement,  the  clays  that 
gathered  in  the  deeper  and  quieter  waters  in  mid-lake  being 
inclosed  by  the  sand  and  gravel  that  accumulated  about  the 
shallower  lake  borders.  (2)  Except  about  the  borders,  where 
beach  and  delta  structures  may  occur,  beds  laid  in  lakes  are 
horizontal  and  often  essentially  uniform  in  texture  over  con- 
siderable areas  (Fig.  285).  This  is  in  contrast  with  stream- 


FIG.  285.  —  Lake-laid    clays    and  \fine    sands.     Near    Devils    Lake,    Wis. 

(Trowbridge.) 

laid  beds  (p.  40).  (3)  The  beds  may  be  bordered  by  shore 
features,  such  as  spits,  bars,  cliffs,  and  beach  ridges,  although, 
save  in  arid  regions,  such  features  are  soon  destroyed  by  stream 
erosion.  (4)  Where  fossils  occur,  they  often  indicate  the 
character  of  the  water  body  in  which  the  associated  sediments 
were  deposited,  since  the  life  inhabiting  fresh,  brackish,  and 
salt  water  differs.  (5)  Broad  flats  and  mountain  meadows 
may  be  so  related  to  the  inclosing  slopes  as  to  indicate  clearly 
that  they  are  the  beds  of  former  lakes. 

Certain  noted  extinct  lakes  are  referred  to  later  (pp.  453. 
458). 


268  PHYSICAL  GEOLOGY 

NONFRAGMENTAL   DEPOSITS   IN    LAKES 

The  more  important  points  concerning  the  distribution 
and  structure  of  lake-laid  beds  of  gravel,  sand,  mud,  etc., 
are  indicated  above.  The  leading  chemical  and  organic  de- 
posits of  lakes  may  be  described  further. 

Salt  lakes.  —  Many  lakes  in  arid  regions  lose  as  much  or 
more  water  by  evaporation  into  the  dry  air  than  they  receive 
as  rain  on  their  surfaces  and  as  run-off  from  the  tributary 
slopes.  Such  a  lake  cannot  maintain  an  outlet.  Streams 
bring  minerals  into  the  lake,  often  calcium  carbonate, 
gypsum,  and  common  salt,  which  have  been  dissolved  during 
the  passage  of  the  water  through  or  over  the  rocks.  As 
water  is  evaporated  from  the  surface  of  the  lake,  these  things 
are  left  behind,  and  as  the  process  continues,  the  waters  of 
the  lake  become  more  and  more  saline.  When  they  become 
over-saturated,  the  minerals  begin  to  be  precipitated  from 
solution.  The  deposition  of  calcium  carbonate  often  pre- 
cedes that  of  gypsum,  which  is  followed  in  turn  by  common 
salt.  Certain  salt  lakes  and  seas,  such  as  the  Caspian  and 
Aral  seas,  represent  portions  of  former  arms  of  the  ocean, 
now  isolated  by  diastrophism.  Such  lakes  begin  their 
careers  with  the  saltness  of  the  sea.  Most  salt  lakes,  how- 
ever, owe  their  salinity  to  the  gradual  concentration  of  salt 
leached  by  ground  waters  from  the  rocks  and  brought  in  by 
streams. 

One  hundred  pounds  of  average  sea  water  contain  nearly 
3J  pounds  of  mineral  matter  in  solution,  of  which  more 
than  three  fourths  is  common  salt.  The  waters  of  many 
lakes  are  much  salter  than  this.  Those  of  Great  Salt  Lake, 
for  example,  contain  about  18  per  cent  by  weight  of  dis- 
solved salts.  This  dissolved  material  is  chiefly  common  salt, 
of  which  the  lake  is  estimated  to  contain  some  400,000,000 
tons.  Lake  Van,  in  eastern  Asiatic  Turkey,  contains  33  per 
cent  of  salt,  and  is  the  densest  lake  known. 

Extensive  salt  beds  which  were  deposited  in  ancient  lakes 


OCEANS  AND  LAKES  269 

or  arms  of  the  sea  are  found  interbedded  with  other  rocks 
in  various  regions.  Those  in  central  New  York  may  have 
an  area  underground  of  some  10,000  square  miles  (larger 
than  Vermont),  and  individual  beds  are  in  places  80  feet 
thick.  To  form  a  layer  of  salt  80  feet  in  thickness  would 
require  the  evaporation  of  some  6000  feet  of  sea  water  of 
average  salinity.  Clearly,  the  climate  of  New  York  must 
have  been  much  less  humid  than  now  at  the  time  the  salt 
was  deposited.  Salt  beds,  like  other  deposits,  therefore  aid 
in  determining  the  geography  of  the  past.  Their  presence 
points  to  a  period  of  aridity ;  their  thickness  suggests  some- 
thing of  its  duration. 

Other  chemical  deposits.  —  While  the  deposits  mentioned 
in  the  preceding  paragraphs  are  perhaps  the  most  common 
ones,  many  other  substances  are  precipitated  from  the 
waters  of  certain  lakes.  For  example,  iron  is  precipitated 
so  abundantly  in  some  of  the  lakes  of  Sweden  that  it  is  of 
commercial  value,  and  in  the  colonial  period  it  was  dredged 
from  certain  of  the  morainic  lakes  in  eastern  Massa- 
chusetts. 

Marl.  —  Marl  is  a  soft,  limy  clay,  formed  principally  on 
lake  bottoms.  The  calcium  carbonate  is  contributed  by 
the  shells  of  fresh-water  mollusks,  by  the  decay  of  lime- 
secreting  lake  plants,  and  possibly  in  some  instances  by 
chemical  precipitation.  Only  where  little  clay  is  washed 
from  the  slopes  tributary  to  the  lake  is  marl  formed. 
Marl  is  used  extensively  for  the  manufacture  of  Portland 
cement. 

Peat.  —  The  formation  of  peat  in  flood-plain  marshes  has 
been  noted  (p.  178).  Extensive  peat  deposits  have  been 
made  also  in  shallow  lakes  and  in  the  marshes  which  in 
many  cases  replace  them.  Peat  deposits  are  most  ex- 
tensive in  regions  having  moist  and  relatively  cool  climates, 
though  by  no  means  confined  to  them.  A  moist  climate 
favors  a  heavy  vegetation,  and  a  cool  climate  retards  its 
decay. 


PLATE  XV.  A  SECTION  OF  THE  CALIFORNIA  COAST,  A  FEW  MILES 
SOUTH  OF  SAN  FRANCISCO.  Contour  interval,  25  feet.  Scale  about  1  mile 
per  inch.  (San  Mateo,  California,  Sheet,  U.  S.  Geological  Survey.) 


OCEANS  AND  LAKES 


271 


5    / 


QUESTIONS 

1.  How  may  the  fact  that  a  given  beach  grades  regularly  from 
coarse  gravel  at  one  end  to  fine  sand  at  the  other  be  explained  ? 

2.  Why   is   there   generally   a   larger   percentage    of   elliptical 
pebbles  along  beaches  than  along  stream  beds? 

3.  (1)  What  was  the  origin  of 
the  long,  narrow  peninsula  shown 
in   Figure  286?     What  is  it 
called  ?    Why  does  it  not  extend 
entirely   across    the    bay?      (2) 
What  is  the  prevailing  direction 
of  the  littoral  currents  along  this 
shore  ?    How  is  it  shown  ?     (3) 
Account     for      the     triangular 
marshy  tract  on  the  northeastern 
side  of  the  bay.    (4)  What  seems 
likely  to  be  the  future  history  of 
the  bay  ?    What  are  the  several 
agencies  which   will  assist  in 
bringing  it  about? 

4.  Plate  XV.     (1)  Where 
along  this   shore   is   erosion  in 
progress  ?    (2)  Is  deposition  any- 
where  in  progress  ?     (3)   What 
is  the   probable  explanation  of 
the  irregularities  of  the  coast  line 


Scale  of  Miles 

I      i/i     2 


FIG.  286.  —  Map  of  Morro  Bay,  coast 

of  California, 
in  the  vicinity  of  Devil's  Slide? 

(4)  Why  are  parts  of  the  coast  high,  and  other    portions   low? 

(5)  What  was  the  probable  origin  of  the  island  off  San  Pedro  Point  ? 

(6)  Explain  Lake  Mathilde  and  Laguna  Salada.     (7)  What  changes 
may  be  expected  in  the  character  of  this  coast  line  in  the  future  ? 

5.  What  inferences  may  be  made  from  the  fact  that  the  beach 
lines  of  certain  extinct  lakes  are  not  horizontal? 

6.  (1)  Why  is  the  ocean  salt  ?     (2)  Why  are  broad,  shallow  lakes 
more  likely  to  become  salt  than  deep,  narrow  ones  ? 

7.  The  size  of  certain  salt  lakes  is  decreasing.     Does  it  follow 
(assuming  that  the  climate   remains  the    same)  that    these  lakes 
will  finally  dry  up  ? 

8.  Account  for  the  fact  that  borings  about  salt  lakes  have  often 
shown  layers  of  salt  alternating  repeatedly  with  layers  of  clay. 

9.  Are  the  waters  of  coastal  lakes  that  are  separated  from  the 
ocean  by  bars  generally  fresh  or  salt  ?     Why  ? 


272  PHYSICAL  GEOLOGY 


REFERENCES 

THE  OCEAN 

AGASSIZ,  A. :   Three  Cruises  of  the  Blake.     (Cambridge,  1888.) 
BONNEY  :    The  Work  of  the  Ocean,  in  The  Story  of  Our  Planet,  pp. 

168-190. 

Challenger  Expedition  Reports.     (London,  1880-1895.) 
DANA  :    Corals  and  Coral  Islands.     (London,  1875.) 
FLINT  :    A  Contribution  to  the  Oceanography  of  the  Pacific;    Smith- 
sonian Institution,  Bull.  No.  55.      (Washington,  1905.) 
GEIKIE,  Sir  A.  :    The  Sea  and  its  Work  on  the  Scottish  Coast-Line,  in 

Scenery  of  Scotland,  Ch.  III.     (London,  1901.) 
GEIKIE,  J.  :   Coast-Lines,  in  Earth  Sculpture,  Ch.  XV.     (New  York, 

1898.) 
GILBERT  :    The  Topographic  Features  of  Lake  Shores,  in  5th  Ann. 

Kept.,  U.S.  Geol.  Surv.,  pp.  69-123. 
GULLIVER  :    Shoreline   Topography,  in  Proc.  Am.   Acad.  Arts  and 

Sci.,  Vol.  XXXIV,  pp.  151-252. 
MURRAY  :    Depth,  Temperature,  Deposits,  etc.,  of  the  Ocean,  in  Scot. 

Geog.  Mag.,  Vol.  XV,  pp.  505-522. 
SALISBURY  :    The  Mineral  Matter  of  the  Sea,  in  Jour,  of  Geol.,  Vol. 

XIII,  pp.  469-484. 
SHALER  :    The  Geological  History  of  Harbors,  in  13th  Ann.  Kept., 

U.S.  Geol.  Surv.,  Pt.  II,  pp.  93-209. 

—  Beaches  and  Tidal  Marshes  of  the  Atlantic  Coast,  in  Physiog- 
raphy of  the  United  States,  pp.  137-168.     (New  York,  1895.) 

—  Sea  and  Land.     (New  York,  1894.) 
THOMSON  :    The  Depths  of  the  Sea.     (London,  1874.) 
WILD  :  Thalassa.     (London,  1877.) 

LAKES 

BRIGHAM  :    Lakes:    a  Study  for  Teachers,  in  Jour,  of  Sch.  Geog., 

Vol.  I,  pp.  65-72. 
DAVIS  :    On  the  Classification  of  Lake  Basins,  in  Proc.  Bost.  Soc. 

Nat.  Hist.,  Vol.  XXI,  pp.  315-381.     (Contains  many  references 

on  subject.) 

-  The  Classification  of  Lakes,  in  Science,  Vol.  X,  1887,  pp.  142~ 
143. 

DILLER  :     The   Geology   and   Petrography   of  Crater   Lake   National 
Park;   Prof.  Paper  No.  3,  U.S.  Geol.  Surv. 

-  Crater  Lake,  Oregon,  in  Nat.  Geog.  Mag.,  Vol.  VIII,  pp.  33-48. 


OCEANS  AND  LAKES  273 

DRYER  :     The  Morainic  Lakes  of  Indiana,  in  Studies  in  Indiana 

Geography,  Ch.  VI.     (Terre  Haute,  1897.) 
FENNEMAN  :  Lakes  of  Southeastern  Wisconsin;  Wis.  Geol.  and  Nat. 

Hist.  Surv.,  Bull.  VIII. 
GILBERT  :   Lake  Bonneville;   Mono.  I,  U.S.  Geol.  Surv. 

-  The  Topographic  Features  of  Lake  Shores,  in  5th  Ann.  Kept., 
U.S.  Geol.  Surv.,  pp.  69-123. 

GOLDTHWAIT  i    The  Abandoned  Shore-Lines  of  Eastern   Wisconsin: 

Wis.  Geol.  and  Nat.  Hist.  Surv.,  Bull.  XVII. 
RUSSELL  :   Lakes  of  North  America.     (Boston,  1895.) 
Present  and  Extinct  Lakes  of  Nevada,  in  Physiography  of  the 

United  States,  pp.  101-132.     (New  York,  1895.) 
Lake  Lahontan;   Mono.  XI,  U.S.  Geol.  Surv. 

-  Mono  Lake,  in  8th  Ann.  Kept.,  U.  S.  Geol.  Surv.,  pp.  269-319. 
SALISBURY  and  KUMMEL  :   Lake  Passaic  —  an  Extinct  Glacial  Lake, 

in  Jour,  of  Geol.,  Vol.  Ill,  pp.  533-560 ;  and  N.J.  Geol.  Surv., 
Report  on  Surface  Geology  for  1893,  pp.  225-328. 

TAYLOR  :  A  Short  History  of  the  Great  Lakes,  in  Studies  in  Indiana 
Geography,  Ch.  X.  (Terre  Haute,  1897.) 

UPHAM  :   The  Glacial  Lake  Agassiz;  Mono.  XXV,  U.S.  Geol.  Surv. 


CHAPTER  VIII 
THE    GREAT   RELIEF   FEATURES    OF   THE   LAND 

MOUNTAINS,  plateaus,  and  plains  have  been  referred  to 
frequently  in  previous  pages.  The  more  important  points 
concerning  their  origin  and  life  history  are  summarized  in 
the  present  Chapter.  Most  of  the  geological  processes  and 
agents  are  concerned  in  their  formation  and  development. 
The  present  characteristics  of  any  given  relief  feature  are 
determined  chiefly  by  (1)  the  method  of  its  formation,  (2)  its 
original  altitude  and  its  distance  from  the  sea,  (3)  the  char- 
acter and  structure  of  its  rocks,  (4)  the  character  of  the 
agents  of  erosion  that  are  slowly  wearing  it  down  and  the 
conditions  which  govern  their  work  (especially  the  nature 
of  the  climate),  and  (5)  the  stage  in  their  work  which  the 
erosive  agents  have  reached. 

MOUNTAINS 

A  mountain  is  an  elevation  which  rises  prominently  above 
the  surrounding  country,  and  which  has  a  comparatively 
small  area  on  top  (Fig.  287).  On  a  low,  flat  plain,  a  moun- 
tain may  have  a  height  of  only  a  few  hundred  feet ;  in  more 
rugged  surroundings,  a  much  higher  elevation  may  be  called 
a  hill.  The  matter  is  therefore  a  relative  one,  and  no  fixed 
height  is  necessary  in  order  that  an  elevation  may  be  classed 
as  a  mountain.  Certain  plains  —  for  example,  the  western 
portion  of  the  Great  Plains  —  are  higher  than  many  moun- 
tains, —  for  instance,  the  northern  Appalachians  (Figs.  43  and 
44,  pp.  62  and  63).  Most  mountains,  however,  are  higher 
than  most  plateaus  and  plains.  A  few  of  them  reach  eleva- 
tions above  sea  level  of  nearly  30,000  feet,  or  about  5^  miles, 

274 


THE  GREAT  RELIEF  FEATURES  OF  THE  LAND     275 

Compared  with  the  diameter  of  the  earth,  even  the  loftiest 
mountains  are  insignificant  protrusions  of  the  lithosphere. 

Mountain  ridges  and  peaks  are  grouped  commonly  in 
relatively  long  and  narrow  belts,  called  mountain  ranges. 
When  several  more  or  less  parallel  ranges  are  grouped  to- 
gether, they  constitute  a  mountain  system.  Thus,  one  speaks 
of  the  Wasatch  Range  of  Utah,  but  of  the  Rocky  Mountain 
System. 

Distribution  of  mountains.  —  It  is  noteworthy  that  moun- 
tain ranges  are  situated  in  general  near  the  edges,  rather 
than  in  the  interiors,  of  the  land  masses.  It  is  striking,  also, 


FIG.  287.  —  Mountains    rising    conspicuously    above    an    aggraded    plain. 
Alaska.     (Netland,  U.S.  Boundary  Commission.') 

that  most  of  the  loftiest  mountain  chains  are  not  far  from 
the  shores  of  the  greatest  sea,  the  Pacific  Ocean.  In  a  general 
sense,  the  land  masses  accordingly  have  two  very  unequal 
slopes,  a  short  and  relatively  steep  one  toward  the  Indian- 
Pacific,  a  long  and  gentle  one  toward  the  Atlantic  or  Arctic 
Ocean.  What  is  most  significant  probably  is  that  most 
mountain  chains  are  near  the  junctions  of  the  continental 
plateaus  and  the  ocean  basins,  and  that  most  of  the  longest 
and  highest  ones  are  near  the  edges  of  the  greatest  basin. 
The  settling  of  the  larger  and  heavier  ocean  basins,  due  to 
the  cooling  and  consequent  contraction  of  the  earth,  possibly 
may  have  been  an  important  cause  of  the  deformation  of  the 


276  PHYSICAL  GEOLOGY 

edges  of  the  smaller  and  lighter  continental  plateaus.  The 
matter  is,  however,  an  unsolved  problem. 

The  leading  types  of  mountains  are  noted  below. 

Faulted  (block)  mountains.  —  Figure  288  shows  several 
mountain  ridges,  and  suggests  their  origin.  A  plateau  or 
plain  was  divided  by  fissures  into  a  series  of  great  blocks, 
which  were  displaced  by  faulting,  the  relatively  elevated 
edges  forming  mountain  ridges.  The  mountain  ridges  may 
owe  their  relief  to  their  having  been  uplifted,  or  to  the  sink- 
ing of  the  lower  land,  or  to  both.  Such  mountains  are  called 
faulted  or  block  mountains.  The  mountains  shown  are  still 
young,  for  their  crests  are  without  notches,  and  streams  have 
not  carved  valleys  in  their  even  slopes;  little  talus  has  ac- 
cumulated at  the  base  of  the  great  fault  scarps.  The  smooth 


FIG.  288.  —  Diagram  of  block  mountains. 

slopes  show  also  that  the  surface  from  which  the  mountains 
were  formed  was  essentially  level,  and  therefore  topographic- 
ally either  young  or  old.  The  ridges  diminish  gradually  in 
elevation  from  the  points  where  the  vertical  displacement  of 
the  beds  was  greatest,  and  die  out  where  the  faults  end.  The 
beds  dip  away  from  the  fault  scarps. 

As  time  passes,  streams  will  dissect  the  now  smooth 
slopes.  Later,  the  larger  valley  bottoms  and  finally  even  the 
strongest  inter-valley  spurs  will  be  worn  down  to  base 
level,  unless  the  mountains  are  maintained  by  further  dias- 
trophism.  Buried  beneath  the  waste-mantled  surface  of 
the  resulting  plain,  the  fault  planes  and  tilted  beds  will 
record  the  former  existence  of  the  mountains. 

Young  block  mountains  1000  or  1200  feet  high  and  10 
to  30  or  40  miles  long  occur  in  southern  Oregon  and  the 
adjacent  states.  In  Nevada  there  are  block  mountains  now 


THE  GREAT  RELIEF  FEATURES  OF  THE  LAND     277 

maturely  dissected.  There  are  many  faulted  mountains  in 
the  Great  Basin  region,  where  they  are  perhaps  the  leading 
type  of  mountain  structure  (Fig.  289). 


FIG.  289.  —  Diagram  showing  the  general  arrangement  of  block  mountains 
in  the  Great  Basin.  Each  mountain  block  is  tilted  in  the  direction  indi- 
cated by  the  light  slanting  lines.  The  broken  lines  show  the  profiles  of 
the  mountains  before  erosion.  The  dotted  portions  of  the  diagram  rep- 
resent the  accumulation  of  waste.  Length  of  the  section,  120  miles. 
(Gilbert.) 

Folded  mountains.  —  Mountains  consisting  of  a  series  of 
earth  folds  are  a  common  type.  In  the  Jura  Mountains  of 
France  and  Switzerland,  open,  symmetrical  anticlines  form 
parallel  ridges,  separated  by  synclinal  troughs  (Fig.  290). 
The  structure  of  most  folded  mountains  is  much  more  com- 


FIG.  290.  —  Diagram  showing  symmetrical  folds  of  the  Jura  Mountains. 

plex  than  that  of  the  Juras.  In  parts  of  the  Appalachians, 
for  example,  the  folds  are  closed,  unsymmetrical,  and  often 
overturned,  and  the  structure  is  complicated  by  many  faults, 
some  of  which  have  a  vertical  displacement  of  several  thou- 
sand feet  (Figs.  291  and  292).  Here  the  strata  were  sub- 


FIG.  291.  —  Structure  section  in  the  Appalachian  Mountains.  South- 
western corner  of  North  Carolina.  (From  Nantahala,  N.  C.  —  Tenn., 
Geologic  Folio,  U.S.  Geol.  Surv.) 

jected  to  much  greater  compression  than  in  the  Jura  Moun- 
tains. Most  of  the  strike  faults  in  folded  mountain  regions 
are  overthrusts  (Why?).  Where  the  folding  of  the  beds 
was  intense,  the  larger  folds  are  composed  commonly  of  a 
diminishing  series  of  minor  folds  (Fig.  291),  the  smallest  of 


278 


PHYSICAL  GEOLOGY 


which  may  be  of  microscopic  size.  Compression  tends  also 
to  metamorphose  the  rocks,  and  usually  the  amount  of 
change  which  they  have  undergone  corresponds  to  the  in- 
tensity of  the  folding. 

The  present  topography  of  most  folded  mountains — for 
example,  of  the  Juras  and  Appalachians  —  is  not  controlled  by 


^ 


FIG.  292.  —  Structure  section  in  the  Appalachian  Mountains.  Eastern 
Tennessee,  western  North  Carolina.  (From  Greeneville,  Tenn.  —  N.  C., 
Geologic  Folio,  U.S.  Geol.  Surv.) 

the  original  folding  and  faulting,  but  by  subsequent  erosion 
and  diastrophism.  In  many  cases  the  crests  of  the  anticlines 
were  weaker  than  the  synclines,  for  their  rocks  were  stretched 
by  the  folding,  and  joints  and  other  openings  were  widened, 
while  the  rocks  of  the  synclines  were  compressed  and  strength- 
ened ;  and  so  the  anticlines  have  often  come  to  form  the  valleys, 
while  the  synclines,  originally  the  valleys,  constitute  the 


FIG.  293.  —  Cross  section  of  a  portion  of  the  Appalachian  Mountains,  showing 
synclinal  ridges  and  anticlinal  valleys.     (Rogers.) 

ridges  (Fig.  293).  The  development  of  the  present  topog- 
raphy of  the  Appalachian  Mountains  (Plate  XVI)  was  dis- 
cussed in  a  general  way  on  page  153  in  connection  with  cycles 
of  erosion. 

It  is  particularly  to  be  noted  that  the  formation  of  folded 
mountains,  and  indeed  of  all  mountains,  is  an  extremely 
slow  process,  probably  occupying,  in  the  case  of  the  greater 
ranges,  hundreds  of  thousands  or  even  millions  of  years. 
Many  mountains  appear  to  be  growing  now,  —  for  example 
the  Sierras  and  the  St.  Elias  Range. 


279 


PLATE   XVI.    APPALACHIAN  RIDGES.     Contour  interval,  20  feet.     Scale, 
about  1  mile  per  inch.     (Harrisburg,  Penn.,  Sheet,  U.  S.  Geological  Survey.) 


280 


PHYSICAL  GEOLOGY 


Mountains  produced  by  vulcanism.  —  Many  of  the  highest 
isolated  mountains  are  volcanic  cones  (p.  46).  Fujiyama, 
a  volcanic  mountain  in  Japan  (Fig.  294),  has  an  elevation 
above  sea  level  of  12,365  feet.  Many  volcanic  piles  surpass 
Fujiyama  in  height,  but  few,  if  any,  in  symmetry  of  form. 
Aconcagua,  an  Andean  cone  on  the  border  of  Chile  and 
Argentina,  reaches  22,860  feet. 

Many  mountains  have  been  formed  also  by  massive  in- 
trusions of  lava  which  have  domed  or  lifted  the  overlying 
strata  high  above  the  level  of  the  surrounding  country 


FIG.  294.  —  Cone  of  Fujiyama,  Japan. 

(p.  51).  Frequently  the  cover  of  sedimentary  rocks  has  been 
removed  partially  by  erosion,  exposing  the  central  core  of 
igneous  rocks,  from  which  the  sedimentary  beds  often  dip 
more  or  less  uniformly  in  all  directions.  The  harder  sedi- 
mentary layers,  wearing  down  less  rapidly  than  the  softer 
ones,  may  stand  out  as  rudely  circular  ridges  alternating 
with  race-track  shaped  valleys,  all  of  which  inclose  the 
igneous  center.  The  Henry  Mountains  of  Utah  (p.  50), 
the  Bear  Paws  and  Little  Snowies  of  Montana,  and  the 
Elk  and  Park  ranges  of  Colorado  are  examples  of  this  general 
class  of  mountains. 


THE  GREAT  RELIEF  FEATURES  OF  THE  LAND     281 


Certain  mountains  with  the  general  structure  of  laccoliths 
—  for  example,  the  Adirondack  Mountains  —  are  not  due 
directly  to  the  intrusion  and  doming  effect  of  their  igneous 
cores.  The  original  mountains  formed  by  the  intrusions  were 
worn  away  in  former 
cycles  of  erosion.  The 
present  mountains  are 
residuals  of  strong 
rocks  left  standing  by 
the  removal  of  the 
surrounding  weaker 
rocks. 

Mountains  of  un- 
equal erosion.  —  As 
already  indicated, 
many  mountains  owe 
their  existence  simply 
to  the  superior  resist- 
ance of  their  rocks, 
which  have  remained 
in  bold  relief  after  the 


FIG.  295.  —  A  peak  in  the  Wasatch  Mountains, 
with  large  accumulations  of  talus  about  its 
base.  (R.  T.  Chamberlin.) 


removal  of  the  sur- 
rounding softer  rocks,  or  to  a  favorable  position  among 
drainage  lines.  Such  mountains  are  sometimes  called  moun- 
tains of  drcumerosion  or  circumdenudation.  Most  mountain 
peaks  (aside  from  volcanic  peaks)  are  of  this  origin  (Fig.  295). 
Pikes  Peak,  Colorado,  and  Mt.  Mitchell,  North  Carolina,  the 
highest  peak  in  the  Appalachian  Mountains,  are  notable  ex- 
amples. The  Catskill  Mountains  of  southeastern  New  York 
are  really  a  dissected  plateau.  Any  maturely  dissected  plateau 
of  considerable  relief  might  similarly  be  called  a  group  of 
mountains  (Fig.  296). 

Combination  mountains.  —  Folding  and  faulting,  vulcan- 
ism  and  unequal  erosion,  may  all  be  concerned  in  the  forma- 
tion of  lofty  mountains.  Many  mountainous  regions,  fur- 
thermore, have  had  several  periods  of  growth,  between 


282 


PHYSICAL  GEOLOGY 


FIG.  296.  —  Summits   of   Endicott    Mountains,    Alaska.     Shows    dissected 
plateau  feature.     (Brooks,  U.S.  Geol.  Sun.) 

which  the  upraised  beds  were  much  wasted  by  erosion  (Fig. 
297). 

The  destruction  of  mountains.  —  Unless  renewed  by  dias- 
trophism  or  vulcanism,  all  mountains  are  in  time  destroyed 
by  erosion.  It  is  to  be  noted,  too,  that  the  erosion  of  moun- 
tains commences  as  soon  as  they  begin  to  rise,  and  continues 
throughout  the  long  period  of  their  growth,  as  well  as  after- 
wards. Accordingly,  no  mountain  due  to  vulcanism  or 


FIG.  297.  —  Diagram  showing  structure  of  the  beds  in  the  region  of  the 
Santa  Lucia  Range,  Cal.  (From  San  Luis,  Cal.,  Geologic  Folio,  U.S. 
Geol.  Surv.) 

diastrophism  ever  had  the  full  height  which  those  processes 
would  have  given  it,  if  unopposed  by  erosion.  As  already 
pointed  out,  many  mountains  have  been  several  times  nearly 
or  wholly  reduced  and  revived  again.  The  length  of  the 
life  of  a  mountain  which  has  ceased  to  grow  is  determined 
largely  by  its  height,  by  the  resistance  of  it's  rocks,  and  by  the 
character  of  the  climate.  Where  mountain  slopes  are  steep, 
as  is  likely  to  be  the  case,  erosion  is  rapid,  and  the  products 


THE  GREAT  RELIEF  FEATURES  OF  THE   LAND     283 


of  weathering  ure  removed  promptly. 
The  higher  slopes  of  many  moun- 
tains are  accordingly  of  bare  rock 
(Fig.  298).  Daily  changes  in  the 
temperature  of  the  rocks  are  greater 
in  the  high  altitudes  of  lofty  moun- 
tains than  in  lower  situations.  Rock 
splitting  is  therefore  important,  and 
where  the  slopes  are  not  too  steep, 
mountains  may  be  covered  with 
loose,  angular  fragments  broken  from 
the  rocks  beneath  (Figs.  299  and 
95).  Mountains  receive  greater 
rainfall  than  plains,  and  even  in 
arid  regions  their  steep  valleys  may 

contain  rushing  torrents.  Rapid  cutting  is  sometimes  op- 
posed, however,  by  the  fact  that  the  streams  are  clear 
(Why?).  The  absence  of  protecting  vegetation  on  many 
upper  mountain  slopes  is  important  (How?).  Wind  veloci- 
ties are  often  great  about  mountain  heights,  but  generally 


FIG.  299.  —  Crumbling  on  a  peak  in  the  Bighorn  Mountains,  Wyo.      (Trow- 

bridge.) 


284 


PHYSICAL  GEOLOGY 


wind  work  is  hindered  through  lack  of  suitable  tools.     Many 
lofty  mountains  are  being  reduced,  also,  by  glaciers.     In  many 

.    other  mountains  with  few 

or  no  glaciers  at  present, 
the  striking  results  of  the 
work  of  former  glaciers 
maybe  seen  (p.  219).  In 
many  cases  neighboring 
glaciers  drove  their  val- 
ley sides  and  cirques  back 
into  the  mountain  mass 
until  they  were  separated 
only  by  sharp  crests  and 
narrow  ridges  (Fig.  300). 
Mountain  scenery  is  due 
far  more  to  the  agents  of 
land  sculpture  than  to  the 
forces  of  diastrophism. 

The  subdued  and  gentle 
slopes  of  the  later  life  of 
a  mountain  are  worn  less 
rapidly  than  the  steeper 
slopes  of  its  earlier  career, 
so  that  its  old  age  is  likely 
to  be  longer  than  its  youth 
and  maturity  combined. 
All  lofty  mountains  are 
comparatively  young,  geologically  speaking;  if  old,  they 
would  have  been  worn  low.  While  very  old  mountains  are 
low,  obviously  not  all  low  mountains  are  old. 


FIG.  300.  —  Serrate  mountain  peaks  due 
to  erosion  by  valley  glaciers.  (Trow- 
bridge.) 


PLATEAUS 

A  plateau  is  a  relatively  elevated  area  of  comparatively 
flat  land,  which  is  commonly  limited  on  at  least  one  side  by 
an  abrupt  descent  to  lower  land.  While  plateaus  are  usually 


THE  GREAT  RELIEF  FEATURES  OF  THE  LAND     285 

higher  than  plains,  they  may  be  lower.  Though  the  ideal 
plateau  has  a  level  surface,  many  are  deeply  trenched  by 
valleys  and  surmounted  by  ranges  of  high  hills  or  mountains. 

Distribution.  —  Extensive  plateaus  are  confined  largely  to 
three  classes  of  situations.  (1)  They  may  intervene  between 
a  lower  plain  on  one  side,  and  higher  mountains  on  the 
other.  The  Piedmont  Plateau,  separating  the  Atlantic 
Coastal  Plain  from  the  Appalachian  Mountains,  is  an  exam- 
ple. (2)  They  may  be  surrounded  more  or  less  completely 
by  mountains,  as  in  the  case  of  the  vast  plateaus  of  Central 
Asia  and  the  Great  Basin  of  western  United  States.  (3)  In 
some  cases  they  rise  abruptly  from  the  sea,  or  from  narrow 
coastal  plains.  The  Iberian  Peninsula  and  southern  India 
are  plateaus  of  this  type. 

Origin  of  plateaus.  —  Plateaus  may  originate  in  various 
ways.  They  may  be  built  by  successive  lava  flows,  like  the 
Columbian  Plateau  of  the  Northwest  and  the  Deccan  Plateau 
of  India.  The  adjacent  country  may  have  been  worn  low 
or  warped  down,  leaving  a  table-land.  Or,  the  plateau  may 
have  been  warped  or  faulted  above  its  surroundings. 

The  erosion  of  plateaus.  —  Like  mountains,  all  plateaus 
will  be  worn  down  to  lowlands  if  not  renewed.  Mature 
plateaus  are  table-lands  completely  dissected  by  streams; 
the  original  flattish  surface  has  been  carved  into  hills  and 
valleys,  and  slope  and  relief  are  at  a  maximum.  It  is  at 
this  stage  that  the  arrangement  of  strong  and  weak  rocks 
expresses  itself  most  completely  in  the  details  of  the  topog- 
raphy. Such  regions  have  so  largely  lost  their  plateau 
character,  that,  as  in  the  case  of  the  Catskills  (p.  281),  they 
are  sometimes  called  mountains.  In  one  sense  there  are 
no  old  plateaus,  for,  when  worn  low,  they  constitute  plains. 

PLAINS 

Tracts  of  comparatively  level  and  low  land  are  plains. 
The  term  is  used  loosely,  however,  for  there  are  hilly  and 
B.  &  B.  GEOL.  — 16 


286  PHYSICAL  GEOLOGY 

rolling  plains  as  well  as  level  ones,  and  high  plains  (Fig.  301) 
as  well  as  low  ones.  Great  plains  are  commonly  terminated 
upon  one  or  more  sides  by  an  abrupt  ascent  to  table-lands 
or  mountains. 

Origin  and  classes  of  plains.  —  Various  types  of  plains 
have  been  discussed  in  previous  pages.  Rivers  make  flood 
plains  (p.  176),  delta  plains  (p.  185),  and  peneplains  (p.  147). 
The  ancient  ice  sheets  covered  the  more  or  less  rough  pre- 
glacial  topography  of  extensive  areas  of  northern  United 


FIG.  301.  —  A  typical  view  on  the  high  plains  of  western  Kansas.     (Gilbert, 
U.S.  Geol.  Surv.) 

States  with  drift,  forming  drift  plains  (p.  451).  The  floors 
of  extinct  lakes  form  many  flat  lake  plains,  especially  in 
glaciated  regions  (p.  453).  Most  lake  plains  are  small. 

Great  plains,  like  the  Atlantic  and  Gulf  coastal  plains  of 
the  United  States  and  the  vast  interior  plain  which  stretches 
from  the  Appalachians  to  the  Rockies  (Figs.  43  and  44,  pp. 
62  and  63),  cannot  usually  be  put  in  any  of  the  above 
classes.  Rather,  they  commonly  contain  many  smaller  plains 
of  several  or  all  of  the  types  enumerated.  In  general,  exten- 
sive coastal  plains  are  former  marginal  sea  bottoms,  exposed 
either  by  elevation  of  the  land  or  by  lowering  of  the  surface 
of  the  sea.  Coastal  plains  may  also  be  peneplains,  or  the 


THE  GREAT  RELIEF  FEATURES  OF  THE  LAND     287 

result  of  the  filling  of  a  shallow  sea  border  by  wash  from  the 
land.  Great  interior  plains  are  either  areas  once  high  but 
now  worn  low,  or,  oftener,  they  are  former  coastal  plains 
now  separated  from  the  sea  by  newer  land. 

The  surfaces  of  plains  due  to  recent  emergence  are  likely 
to  correspond  approximately  with  the  bedding  planes  of 
the  strata,  and  the  rocks  are  usually  poorly  consolidated 
(p.  37).  The  surfaces  of  peneplains  developed  on  stratified 
rocks,  on  the  other  hand,  bevel  the  edges  of  beds  without 
regard  to  the  dip,  and  the  rocks  below  the  mantle  rock  are 
generally  well  consolidated. 

The  changes  produced  on  plains  by  erosion  have  been 
discussed  sufficiently  in  earlier  chapters. 

REFERENCES 

DANA:    On  the  Origin  of  Mountains,  in  Am.  Jour.  Sci.,  3d  Series, 

Vol.  V,  pp.  347-350,  423-443  ;  Vol.  VI,  pp.  6-14,  104-115,  161- 

172 
DAVIS  :    The  Ranges  of  the  Great  Basin,  in  Science,  N.  S.,  Vol.  XIV, 

pp.  457-459. 
The  Mountain  Ranges  of  the  Great  Basin,  in  Bull.  Harvard 

Mus.  of  Comp.  ZooL,  Vol.  XLII,  pp.  129-177. 
• An  Excursion  to  the  Plateau  Province  of  Utah  and  Arizona,  in 

Bull.  Harvard  Mus.  of  Comp.  ZooL,  Vol.  XLII,  pp.  1-50. 

—  The   Mountains  of  Southernmost  Africa,  in  Bull.  Am.   Geog. 
Soc.,  Vol.  XXXVIII,  pp.  593-623. 

DUTTON  :    The  Geology  of  the  High  Plateaus  of  Utah;    U.S.  Geog. 

and  Geol.  Surv.,  Rocky  Mtn.  Region.     (Washington,  1880.) 
GILBERT  :    Origin  of  the  Physical  Features  of  the   United  States,  in 

Nat.  Geog.  Mag.,  Vol.  IX,  pp.  308-317. 
JOHNSON  :     The  High   Plains  and   Their    Utilization,  in  21st  Ann. 

Kept.,  U.S.  Geol.  Surv.,  Pt.  IV,  pp.  601-741. 
LE  CONTE  :    Theories  of  the  Origin  of  Mountain  Ranges,  in  Jour,  of 

Geol.,  Vol.  I,  pp.  543-573. 
POWELL  :  Types  of  Orographic  Structure,  in  Am.  Jour.  Sci.,  3d  Series, 

Vol.  XII,  pp.  414-428. 

—  Physiographic  Features,  in  Physiography  of  the  United  States, 
pp.  33-64.     (New  York,  1895.) 

—  Physiographic  Regions  of  the  United  States,  Idem,  pp.  65-100. 
READE  :    The  Origin  of  Mountain  Ranges.     (London,  1886.) 


288  PHYSICAL  GEOLOGY 

RUSSELL  :    A   Geological  Reconnaissance  in  Southern  Oregon,  in  4th 

Ann.  Kept.,  U.S.  Geol.  Surv.,  pp.  431-464. 
TABR  :    Physical  Geography  of  New  York  State,  Chs.  II,  III.     (New 

York,  1902.) 
WILLIS  :     The  Mechanics  of  Appalachian  Structure,  in  13th  Ann. 

Kept.,  U.S.  Geol.  Surv.,  Pt.  II,  pp.  211-283. 
Folios  of  U.S.  Geol.  Surv.  of  areas  in  mountainous  districts. 

A  number  of  the  references  under  other  headings,  especially 
"The  Work  of  Streams"  and  "The  Composition  of  the  Earth,"  are 
serviceable  also  here. 


PART   II 

HISTOKICAL   GEOLOGY 

CHAPTER  IX 
HISTORY  OF    THE   EARTH 

Geologic  and  human  history  compared.  —  By  the  word 
history  we  usually  understand  the  story  of  men,  nations,  or 
civilizations.  In  reality  this  is  only  one  kind  of  history, 
only  a  phase  of  a  much  larger  history,  which  deals  not  only 
with  the  human  race,  but  with  all  other  animals,  with  plants, 
and  with  the  earth  itself  back  to  the  time  of  its  beginning. 
The  history  of  the  earth  and  its  inhabitants  represents  a 
vastly  greater  length  of  time  than  human  annals,  but  much 
less  is  known  about  it.  It  is  impossible  to  say  just  how  long 
the  earth  has  existed ;  but  if  we  measure  the  existence  of  the 
human  race  in  thousands  of  years,  the  duration  of  the  earth 
must  be  reckoned  at  least  by  tens  or  hundreds  of  millions  of 
years.  While  such  enormous  figures  are  quite  too  vast  for 
comprehension,  we  may  gain  some  idea  of  the  length  of  time 
involved  when  we  learn  that  although  the  great  mountain 
ranges  of  to-day  have  remained  almost  unchanged  since  the 
dawn  of  human  history,  others  like  them  have  been  built  up 
and  then  entirely  worn  away,  time  after  time,  even  during  the 
later  portions  of  geologic  history.  The  stupendous  canon 
of  the  Colorado  River  in  Arizona  seems  to  our  eyes  one  of 
the  fixed  and  everlasting  features  of  the  earth ;  yet  it  has  all 
been  made  in  a  very  recent  period  of  the  earth's  existence. 

How  geologic  history  is  worked  out.  —  The  threads  of 
human  history  have  been  gradually  pieced  together  from 


290  HISTORICAL  GEOLOGY 

written  records,  old  monuments  and  ruins,  legends,  and  such 
things.  Of  geologic  events  there  is  almost  no  written  record. 
We  must  depend  upon  (1)  the  testimony  of  facts  afforded  by 
the  study  of  the  rocks,  and  (2)  our  ability  to  interpret  them 
according  to  the  natural  laws  which  control  everything  that 
happens  in  the  universe. 

Record  of  physical  changes  in  the  earth.  —  When  care- 
fully studied,  the  rocks  may  be  made  to  tell  much  about  their 
own  history.  Take,  for  example,  the  conditions  represented  in 
the  diagram  (Fig.  302).  It  is  evident  that  the  lower  rocks 
have  been  folded,  whereas  the  upper  layers  have  not.  Moun- 
tains were  doubt- 
less formed  by  the 
folding,  but  the  un- 
conformity shows 

FIG.  302.  —  Ideal  section  of  sedimentary  rocks.        that      they      W6re 

worn  away,  leav- 
ing a  nearly  flat  surface  upon  which  the  sand,  which  was  to 
become  the  sandstone,  was  laid  down.  The  sandstone  is  evenly 
stratified,  as  if  the  sand  had  been  assorted  by  currents  of 
water;  and  if  the  shells  of  marine  animals  are  found  embedded 
in  the  sand,  they  indicate  that  the  sand  was  spread  out  on  the 
bottom  of  the  sea.  If  the  sandstone  is  now  hundreds  of  feet 
above  the  sea  and  many  miles  inland,  as  is  true  in  many  cases, 
it  indicates  that  notable  changes  have  taken  place  in  the  dis- 
tribution of  land  and  sea  and  in  the  height  of  the  land  above 
the  sea.  From  a  single  section  of  rock  it  may  thus  be  possible 
to  learn  much  that  has  happened  in  its  vicinity  in  the  ages  long 
past.  From  many  such  fragmentary  records  as  this,  a  fairly 
continuous  story  of  changes  of  land  and  sea,  mountain  growths, 
base  leveling,  and  other  events  has  been  worked  out,  espe- 
cially for  the  latter  part  of  the  earth's  history. 

Record  of  changes  in  living  things.  —  On  first  thought  the 
different  kinds  of  animals  and  plants  about  us  seem  as  change- 
less as  the  hills  and  continents.  Each  bird  and  each  tree,  for 
example,  produces  young  almost  exactly  like  itself,  so  that  the 


HISTORY  O      THE  EARTH  291 

kind  seems  to  be  permanent.  Yet  we  know  that  by  skillful 
manipulation  a  new  variety  of  fruit  tree  or  a  new  breed  of 
poultry  may  be  produced  from  the  original  familiar  forms. 
Such  changes  have  been  going  on  under  natural  conditions 
through  all  the  ages  since  life  began,  though  more  slowly; 
and  these  changes,  like  the  slow  wear  of  the  rivers  and  waves, 
have  brought  about  great  transformations. 

If  we  examine  the  most  recently  formed  strata  of  mud  and 
sand,  we  may  find  in  them  the  shells  of  oysters,  corals,  and 
other  water-inhabiting  animals  hardly  to  be  distinguished 
from  those  which  live  in  the  ocean  today.  But  in  much  older 
rocks  the  shells  we  might  discover  would  bear  only  a  faint 
resemblance  to  the  existing  species.  All  of  the  species  of 
animals  which  lived  when  those  sediments  were  being  de- 
posited may  have  long  since  become  extinct  and  have  been 
replaced  by  newer  types  more  like  those  of  the  present. 
The  shells  and  other  traces  of  animals  and  plants  preserved 
in  the  rocks  are  called  fossils;  and,  as  we  shall  see,  fossils  are 
of  great  importance  because  they  enable  us  to  trace  the  prog- 
ress of  life  from  one  stage  of  its  existence  to  another.  When 
geologists  first  discovered  that  in  a  given  series  of  strata  the 
fossils  in  the  lower  layers  were  distinctly  different  from  those 
in  the  upper,  they  supposed  that  the  earlier  animals  had  been 
destroyed  by  some  great  catastrophe,  and  that  a  new  set  had 
then  been  created  to  replace  them.  Soon,  however,  it  became 
clear  that  the  animals  in  the  upper  beds  were  distinctly  related 
to  those  in  the  lower,  and  that  the  differences  between  them 
are  chiefly  matters  of  degree.  The  belief  thus  became  estab- 
lished that  the  modern  animals  and  plants  have  descended 
from  older  and  older  species  by  a  series  of  very  slow  changes 
occupying  millions  of  years. 

Each  individual  animal  and  plant  to-day  begins  its  existence 
as  a  single  minute  cell,  which,  as  it  grows,  divides  and  sub- 
divides many  times  until  it  produces  the  many  cells  which 
the  adult  body  contains.  There  is  every  reason  to  believe 
that,  like  the  individual  animal,  the  whole  animal  kingdom  has 


292  HISTORICAL  GEOLOGY 

grown  from  very  simple  types,  which  were  but  single  micro- 
scopic cells  of  living  substance. 

GROUPS  OF  ANIMALS  AND  PLANTS  1 

The  common  forms  of  plants  and  animals.  —  Every  one 
recognizes  in  a  general  way  the  difference  between  live  things 
and  inanimate  objects,  and  it  is  not  difficult  to  perceive 
certain  fundamental  traits  which  distinguish  the  one  from 
the  other.  We  say  the  tree  is  alive  because  it  grows  by 
taking  into  itself  new  substance,  produces  seed  which  makes 
new  trees,  and  finally  dies.  Granite  cannot  grow,  in  the  same 
sense,  nor  can  it  produce  other  granites;  it  is  not  alive.  Sim- 
ilarly, among  living  things  themselves,  we  can  usually  dis- 
tinguish at  sight  plants  from  animals,  birds  from  fishes, 
mosses  from  grasses,  etc.,  but  often  without  being  able  to 
tell  just  what  the  differences  are.  The  following  descriptive 
outline  will  help  to  make  plain  the  distinctions  between  the 
familiar  groups  of  living  things  and  will  also  impart  some 
acquaintance  with  others  which  are  now  rare  or  extinct,  and 
hence  are  unknown  from  everyday  observation.2 

Plants.  —  Plants  assimilate  food,  grow,  and  reproduce  their 
kind ;  but  most  of  them  do  not  seem  to  feel,  nor  can  they 
move  about  at  will.  They  have  the  ability  to  use  as  food, 
not  only  water,  but  carbon  dioxide  from  the  air  and  certain 
materials  dissolved  in  the  water.  The  vast  majority  of  them 
are  green  in  color.  There  is  scarcely  any  single  feature,  how- 
ever, which  will  serve  to  distinguish  all  plants  from  animals. 

1  The  following  summary  of  the  more  important  groups  of  living  things 
is  inserted  here  to  aid  the  large  number  of  students  of  geology  who  have 
little  or  no  acquaintance  with  biologic  science.     It  is  necessarily  very  brief. 
For  additional  information  reference  may  be  made  to  any  of  the  more  recent 
textbooks  of   zoology  and  botany,  or  to   the  Zittel-Eastman  Textbook  of 
Paleontology. 

2  Where  possible  the  common  English  names  for  the  different  groups  have 
been   used,  but   inasmuch  as   some   of   the  divisions   have    only  scientific 
names,  usually  of  Latin  or  Greek  origin,  it  has  been  necessary  to  make  use 
of  them. 


HISTORY  OF  THE   EARTH  293 

(1)  ALG.E,  FUNGI,  BACTERIA,  ETC.  (Thallophytes). 

The  simplest  of  all  plants.  Among  them  are  seaweeds, 
diatoms,  molds,  yeast,  mushrooms,  etc.  They  have 
no  distinct  roots  or  leaves,  and  they  are  reproduced, 
not  by  seeds,  but  by  minute  germs  or  spores.  Some 
merely  divide,  each  part  then  becoming  a  distinct  plant. 
The  majority  of  thallophytes  live  in  the  water  or  in 
very  moist  places,  and  some  are  mere  single  cells  of 
jellylike  substance,  too  small  to  be  seen  with  the  naked 
eye.  The  fungi,  bacteria,  and  many  others  are  not 
green  in  color. 

(2)  Moss  GROUP  (Bryophytes). 

More  advanced  plants  than  the  last,  in  that  some  of 
them  have  definite  leaves  and  stems.  But  true  seeds 
and  flowers  are  still  lacking.  They  include  the  mosses 
and  liverworts,  —  all  small,  delicate  plants. 

(3)  FERN  GROUP  (Pteridophytes). 

In  the  ferns  and  their  allies  for  the  first  time  we  find 
a  system  of  tubes  and  pores  which  allow  the  circulation 
of  sap  and  air  within  the  plant.  Some  ferns  are  treelike 
and  have  woody  tissues,  but  most  of  them  are  small 
herbs.  Geologically  the  chief  interest  now  centers  in 
a  group  which  we  may  call  the  seed  ferns  (Pterido- 
spermae),1  —  all  of  them  now  extinct.  These  had  all 
the  appearance  of  true  ferns,  and  were  formerly 
regarded  as  such;  but  it  is  now  known  that  they 
possessed  fruitlike  organs  with  true  seeds,  which  indi- 
cate a  close  relationship  with  the  next  group.  They 
seem  to  be  intermediate  in  many  respects  between 
ferns  and  cycads. 

1  The  older  classifications  of  plants  have  recently  been  modified  in  con- 
sequence of  the  studies  of  Seward,  Scott,  and  others  on  fossil  plants.  The 
scheme  here  used  is  adapted  partly  from  Scott's  writings. 


294  HISTORICAL  GEOLOGY 

(4)   SEED  PLANTS  (Spermatophytes). 

This  group,  distinguished  from  all  except  the  transi- 
tional seed  ferns  by  the  production  of  true  seeds,  con- 
tains all  the  higher  plants  which  we  ordinarily  observe, 
as  well  as  many  which  are  extinct.  It  is  divided  into 
two  important  sections:  — 

(a)    Naked-seed  section  (Gymnosperms) . 

The  seeds  are  naked  and  there  are  no  flowers,  in 
the  common  sense  of  that  word.  Here  belong  the 
pines  and  other  "  evergreens,"  as  well  as  the 
cycads,  —  palmlike  plants  now  of  small  impor- 
tance, but  formerly  very  abundant  (Fig.  422). 

(6)   Incased-seed  section  (Angiosperms) . 

Seeds  incased  in  a  husk  or  shell.  This  section 
includes  the  majority  of  our  familiar  trees  and 
shrubs,  such  as  oak,  elm,  apple,  palm,  rose,  etc., 
and  the  grasses,  grains,  and  many  herbs.  To-day 
it  is  the  most  important  group  of  plants,  but  it  was 
the  last  to  appear,  and  in  the  older  geologic  periods 
no  such  types  existed. 

Animals.  —  Animals  differ  so  markedly  from  plants  that 
all  but  the  simplest  forms  may  readily  be  recognized  as  being 
distinct.  They  have  the  power  of  making  voluntary  move- 
ments, they  have  the  sense  of  feeling,  and  they  depend  for 
food  on  the  bodies  of  plants  or  other  animals.  The  higher 
animals  also  possess  the  faculties  of  seeing,  hearing,  etc., 
and  even  of  intelligence,  characteristics  which  separate 
them  more  and  more  widely  from  plants. 

(1)   PROTOZOANS. 

The  simplest  animals  are  so  much  like  the  simplest 
plants  that  only  a  few  things,  such  as  the  power  of 
motion  and  evident  sensation,  distinguish  them. 
Even  these  tests  fail  in  the  lowest  forms.  Most 


HISTORY  OF  THE   EARTH 


295 


of   them    are    minute    beings    which    live    in    water. 
The  jelly  like  amoeba  (Fig.  303),  consisting  of  a  single 


FIG.  303.  — The  Amoeba, 
a  protozoan  without  a 
shell. 


FIG.  304.  —  A  proto- 
zoan with  calcareous 
shell  and  delicate 
threads  of  proto- 
plasm (Globige- 
rina). 


cell,  is  an  example.     The  protozoans  have  no  distinct 

organs,  —  not  even  a  stomach.     Some  are  incased  in 

tiny  chambered  shells,  which  may 

be  preserved  as  fossils  (Fig.  304). 

These   shells   often  make   a  large 

contribution  to  the   formation  of 

limestones. 

(2)   SPONGES. 

They  are  composed  of  many  cells 
but  still  lack  well-defined  organs. 
They  are  provided  with  countless 
pores  and  tubes  through  which 
water  circulates  freely  and  feeds 
each  individual  cell.  They  have  no 
shells,  but  most  of  them  contain 
little  hard  rods  and  spines  (spicules) 
embedded  in  the  flesh  (Fig.  305), 
and  usually  joined  into  a  solid 
framework.  These  spicules  are 
often  found  preserved  in  the  rocks. 


FIG.  305.  — A  simple 
sponge,  opened  to 
show  the  sacklike 
form,  and  with  the 
skin  removed  to 
show  the  tack- 
shaped  spicules 
which  -form  its 
skeleton.  (After 
Haeckel.) 


296          ..-  HISTORICAL  GEOLOGY 

(3)   POLYPS  (Ccelenterates). 

Animals  having  a  central  cavity  which  serves  as  a 
stomach,  and  a  series  of  radiating  arms  or  tentacles 
around  the  mouth.  They  are  all  aquatic  and  most  of 
them  live  in  colonies  attached  to  each  other.  Many 
resemble  plants  in  their  general  appearance  and  sta- 
tionary habits  of  life.  The  classification  of  this  group 
is  complicated,  but  the  following  divisions  are  of  special 
interest. 

(a)  Jellyfish  (Medusce). 

Floating    jellylike    animals    (Fig.    306)    of   great 
beauty  and  interest,  but  de- 
.void  of  hard  parts  and  there- 
fore rarely  preserved  as  fossils. 

(b)  Hydroids. 

Delicate    plantlike     animals, 

most  of  them  without   hard 

parts.    Only  the  extinct  group 

known  as  the  graptolites  are 

common  in   tne  fossil  state.    FlG-  306^— A 'modem 

They  were  minute  polyps  at-       jellyfish  or  medusa. 

tached  in  rows  to  long  stems ; 

in  appearance  the  fossils  resemble  serrate  blades  of 

grass  (Figs.  346  and  347). 

(c)  Corals. 

Most  of  these  polyps  are  provided  with  a  limy 
shell,  exuded  by  the  outer  side  of  the  body,  and 
are  therefore  readily  fossilized.  In  some  species 
each  animal  lives  by  itself  and  leaves  a  horn-shaped 
skeleton  (Fig.  348),  but  the  majority  are  attached 
to  each  other  in  the  form  of  compact  or  branching 
colonies  (Figs.  349  and  372).  The  young  corals 
swim  freely  in  the  water,  but  become  fixed  to  the 
sea  bottom  before  they  reach  maturity. 


HISTORY  OF  THE  EARTH  297 

(4)   ECHI'NODERMS. 

Under  this  name  are  included  such  animals  of  the  sea 
as  starfish,  sea  urchins,  and  crinoids.  They  possess 
distinct  nerves,  digestive  organs,  and  a  circulatory 
system,  in  which  the  fluid  is  water  instead  of  blood. 
These  features  mark  a  notable  advance  over  the 
polyps.  Most  echinoderms  are  protected  by  a  hard 
shell  composed  of  a  mosaic  of  little  plates. 

(a)   Crinoids. 

Often  called  sea  lilies,  because  they  are  attached 

to  the  sea  floor  by  stalks,  while  the  upturned 

mouth     is     surrounded 

by  long,  feathery  arms 

(Fig.  364).  Cystids  and 

blastaids  (Fig.  390)  are 

related  forms. 


(b)  Starfish  (including  the 

brittle  stars). 
On  account  of  their  star- 
like  shape,  these  animals 

Will  not  be  Confused  With      FIG     307. -A     modern     sea 

urchin  with  the  spines  at- 

any  others.  tached. 

(c)  Sea  urchins  (Echinoids). 

Round  or  egg-shaped  forms  which  in  life  are 
covered  with  spines  (Figs.  307  and  444).  They 
are  very  abundant  to-day  along  the  seashores, 
but  are  less  important  among  fossils. 

(5)  WORMS. 

A  large  group  of  animals  which  are  still  more  complex 
in  structure  than  the  echinoderms.  As  they  are  soft- 
bodied  they  are  not  commonly  preserved  in  the  rocks, 
but  their  burrows  in  the  sand  are  often  found  as  tubes 
in  sandstone. 


298  HISTORICAL  GEOLOGY 

(6)  LAMP  SHELLS  (Brachiopods). 

These  animals  are  now  rare  and  are  not  well  known 
even  by  their  English  name.  They  are  inclosed  in  a 
pair  of  shells  (Figs.  369  and  396),  and  hence  are  easily 
confused  with  the  lowest  group 
of  mollusks;  they  may  be  dis- 
tinguished from 
the  latter  by  the 
bilateral  symme- 
try of  their  shells. 
A  pair  of  spiral 

FlG.  308.  —  Internal  structure  of  the  shells       armlike      Organs 
of  brachiopods.  .  , 

aids     in     getting 

food  to  the  mouth.  In  the  earlier  forms  these  spirals 
were  soft  and  hence  were  not  fossilized.  Later,  some 
of  them  came  to  have  solid  supports  of  lime  carbonate 
which  were  durable  (Fig.  308).  Fossil  brachiopods 
are  varied  in  shape  and  are  abundant  in  many  of  the 
older  rocks. 

(7)  MOLLUSKS. 

In  this  group  are  found  not  only  well-developed  internal 
organs,  but,  in  the  higher  types,  even  a  distinct  head, 
eyes,  and  teeth.  All  are  soft-bodied,  but  they  are 
usually  protected  by  a  hard,  limy  shell. 

(a)    Bivalves  (Pelecypods). 

Forms  like  the  common  oyster  and  clam,  provided 
with  two  shells,  usually  nearly  alike,  which  hinge 
together  on  one  side  (Figs.  352  and  374). 

(6)   Snail  group  (Gastropods). 

The  snail  and  its  relatives  have  a  single  conical 
shell  which  is  almost  always  twisted  or  coiled  into 
a  spiral  form.  The  earliest  and  most  primitive 
types  are  merely  cap-shaped  (Fig.  336),  but  later 
species  developed  a  great  variety  of  coiled  shells 


HISTORY  OF  THE   EARTH  299 

(Figs.  350,  365,  and  443).     The  snail-like  mollusks 
live  on  land,  in  fresh  water,  and  in  the  sea. 
(c)    Chambered  mollusks  (Cephalopods). 

The  cephalopods  take  rank  as  the  highest  of  all 
the  mollusks.      They  have  well-developed  eyes, 
and  many  are  active,  voracious  animals. 
i.   Nautilus  division. 

The  animals  of  this  division  are  now  rare, 
although  many  existed  in  earlier  periods.  The 
shell  is  a  long  tube  divided  into  chambers  by  a 
series  of  cross  partitions  (Fig.  353).  A  small 
tube  or  siphuncle  extends  back  through  all  the 
chambers  to  the  apex  of  the  shell.  The  earlier 
shells  were  straight,  but  soon  there  appeared 
curved  (Fig.  354)  and  coiled  forms  (Fig.  355). 
Spiral  shells,  however,  are  rare.  Later  in  the 
history  of  the  cephalopods  the  partitions  be- 
came more  or  less  folded  into  convolutions 
(Fig.  391),  which  are  shown.*  on  the  inner  sur- 
face of  the  shell  by  angles  and  loops  in  the 
lines  (sutures)  formed  by  the  junction  of  the 
partition  with  the  shell.  Eventually  this  fold- 
ing of  the  sutures  became  extremely  complicated 
(Fig.  447). 
ii.  Cuttlefish  division. 

The  cuttlefish,  octopus,  and  squid,  although 
common  enough  to-day,  have  left  fewer  fossils 
than  the  last  group.  Their  stout  bodies,  with 
long,  fleshy  tentacles  surrounding  the  head, 
have  no  hard  parts,  except  that  some  had  a 
cigar-shaped  shell  embedded  in  the  body  of 
the  animal  (Fig.  429). 

(8)   ARTHROPODS  ("Jointed-leg"  animals). 

Besides  less  familiar  types,   this  group  includes  the 
insects,  spiders,  centipedes,  and  crayfish,  —  that  is  to 


300  HISTORICAL  GEOLOGY 

say,  all  the  invertebrates  which  are  provided  with 
jointed  legs.  They  have  well-developed  organs  of 
touch,  sight,  smell,  etc.,  and  their  internal  anatomy 
is  highly  complex. 

(a)  Crustaceans. 

Animals,  usually  aquatic,  the  majority  of  which 
are  covered  by  a  hard  outside  shell  made  "of  a 
number  of  plates.  Here  belong  the  lobsters, 
shrimps,  etc.,  of  to-day,  and  the  even  more  impor- 
tant group  of  trilobites  which  are  now  extinct. 
An  idea  of  the  general  appearance  and  great 
variety  of  the  trilobites  may  be  gained  from 
Figures  333,  334,  344,  and  361. 

(b)  Air-breathing  arthropods. 

The  insects,  spiders,  scorpions,  and  centipedes  are 
common  on  the  land  surface  to-day,  but  fossil 
remains  of  them  are  scarce.  The  animals  of  the 
land  are  apt  to  be  left  in  positions  where  they  are 
less  likely  to  be  preserved  as  fossils  than  are  those 
which  live  in  the  water. 

(9)   VERTEBRATES. 

The  highest  branch  of  the  animals  contains  those  which 
have  a  backbone  or  vertebral  column.  Among  these 
are  the  fishes,  frogs,  reptiles,  birds,  and  four-footed, 
beasts  in  general.  The  subdivisions  of  this  great  group 
we  usually  recognize  without  difficulty. 

(a)   Fishes. 

Fishes  are  the  lowest  of  the  more  familiar  verte- 
brates and  are  the  only  group  which  inhabits  the 
water  exclusively.  They  have  poorly  constructed 
skeletons  which,  in  some  species,  consist  chiefly 
of  cartilage  instead  of  bone.  In  brain  power, 
also,  they  are  the  least  advanced.  They  swim 
by  means  of  fins  and  breathe  water  through  gills. 


HISTORY  OF  THE  EARTH  301 

(b)  Amphibians. 

This  group,  containing  frogs,  salamanders,  etc., 
is  intermediate  between  fishes  and  reptiles.  As 
the  name  indicates,  they  live  partly  in  water  and 
partly  in  air.  The  young,  called  tadpoles  if  frogs, 
breathe  with  gills  and  are  otherwise  much  like 
fishes ;  but  before  they  are  fully  grown  they  usually 
develop  lungs,  shed  their  fins  and  gills,  and  change 
their  habits  accordingly.  Some  amphibians,  how- 
ever, never  relinquish  their  aquatic  habits. 

(c)  Reptiles. 

The  snakes,  crocodiles,  lizards,  turtles,  and  still 
other  forms  now  extinct,  may  at  first  glance  seem 
to  be  like  the  amphibians,  but  in  reality  they  are 
very  different.  They  breathe  air  exclusively  and 
their  skeletons  are  usually  rather  solid  and  well 
constructed.  They  were  formerly  far  more  abun- 
dant and  important  in  the  world  than  they  are  now. 

(d)  Birds. 

The  feathered  vertebrates  are  so  plainly  set  off 
from  all  others  as  to  need  little  description.  In 
spite  of  the  dissimilarity  of  the  two  groups,  it  is 
known  that  the  birds  are  much  more  closely  related 
to  the  reptiles  than  to  any  other  class  (p.  413). 

(e)  Mammals. 

The  common  quadrupeds,  such  as  cattle,  bears, 
mice,  bats,  and  kangaroos,  which  as  a  rule  bring 
forth  live  young  and  nourish  them  with  milk,  are 
now  the  leaders  of  the  organic  world.  Most  of 
them  are  clothed  with  hair  and  inhabit  the  land, 
although  some,  like  the  whales,  are  bare  and  live 
in  the  water.  As  a  group  the  mammals  excel  all 
other  animals  in  intelligence  and  consequently  in 
power,  for  in  all  the  history  of  life,  cunning,  skill, 
and  resourcefulness  have  been  of  more  avail  in 
the  combat  for  existence  than  mere  strength  or  size. 
B.  &  B.  GEOL.  — 17 


302  HISTORICAL  GEOLOGY 

FOSSILS  AND  THEIR  USES 

Preservation  of  fossils.  —  The  traces  of  animals  and  plants 
preserved  in  the  rocks,  and  known  as  fossils,  are  made  in  a 
variety  of  ways.  In  rare  cases  the  entire  animal  is  preserved, 
—  as  when  insects  are  inclosed  in  resin  which  afterwards 
becomes  amber.  More  frequently  only  the  bones  or  shells 
are  left.  Often  the  shells  have  been  dissolved  out  and  other 
mineral  matter  substituted,  giving  us  natural  casts  of  the 
original.  In  still  other  instances  we  have  only  the  markings 
made  by  the  living  animals,  as  the  burrows  of  aquatic  worms 
in  the  sand  or  the  trails  of  clams  in  the  mud.  Whatever  the 
nature  of  these  vestiges  of  life  they  are  all  fossils. 

Manifestly  not  all  animals  or  plants  are  allowed  to  become 
fossils.  The  great  majority  are  devoured,  while  others  decay 
in  the  open  air  and  disappear.  Only  those  which  are  pro- 
tected from  the  atmosphere  are  preserved.  Complete  decay 
is  often  prevented  if  the  animal  or  plant  becomes  lodged  in  a 
bog  or  lake  or  beneath  the  sea.  Since  the  sea  is  far  more 
extensive  than  lakes  or  marshes,  it  is  not  surprising  that  the 
majority  of  fossils  which  have  been  found  are  those  of 
marine  animals.  Correspondingly  there  is  a  scarcity  of 
fossil  remains  of  the  animals  and  plants  which  lived  upon  the 
dry  land. 

Evidence  of  past  conditions.  —  Since  fossils  tell  us  of  the 
living  things  of  bygone  ages,  obviously  they  are  of  interest  to 
the  biologist.  In  geology,  however,  they  have  additional  uses 
apart  from  the  study  of  life  itself.  For  example,  we  may 
find  in  Iowa  a  bed  of  limestone  which  contains  abundant  fossil 
corals  and  other  animals  of  the  sea.  From  this  we  infer  a 
variety  of  things  about  the  conditions  in  the  central  part  of  the 
United  States  in  that  remote  period  when  the  limestone  was 
formed.  Evidently  the  sea  then  extended  far  into  the  interior 
of  the  continent.  That  its  waters  were  shallow  and  rela- 
tively warm  is  shown  by  the  presence  of  the  kinds  of  corals 
which  inhabit  only  the  shallow  portions  of  tropical  seas.  In 


HISTORY  OF  THE   EARTH  303 

brief,  we  may  learn  from  the  fossils  alone  something  about 
the  geography  and  the  climate  of  the  remote  past. 

The  succession  of  faunas.  —  Fossils  also  enable  us  to  tell 
the  relative  age  of  rocks  in  different  parts  of  the  world ;  and 
herein  lies  perhaps  their  greatest  value  to  geology.  Since 
each  successive  bed  of  sediment  is  laid  down  upon  some  other 
which  was  there  before  it,  it  is  obvious  that,  in  any  exposure 
of  undisturbed  rocks,  the  beds  which  lie  below  are  older  than 
those  above.  This  rule  sometimes  fails  to  hold,  as  where 
rocks  have  been  so  highly  folded  that  they  have  been  actually 
bent  back  upon  themselves,  but  such  exceptions  may  with 
care  be  detected  in  the  field.  It  is  evident,  therefore,  that  if 
we  could  find  in  the  sides  of  some  deep  valley,  like  that  of  the 
Colorado  River,  a  complete  pile  of  the  sedimentary  strata 
which  have  been  laid  down  upon  the  earth  since  the  dawn  of 
geologic  history,  we  should  have  a  complete  stone  record  of 
sedimentation.  But  unfortunately  no  section  even  approach- 
ing this  in  completeness  is  known ;  we  have  only  fragmentary 
exposures  more  or  less  concealed  by  soil,  forests,  and  bodies  of 
water.  It  is  here  that  fossils  come  to  our  aid. 

Figure  309  is  drawn  to  represent  two  bare  hills  cut  through 
to  show  the  layers  of  which  they  are  composed.  In  the  hill 
on  the  left  we 
find  the  remains 
of  a  certain  fauna 
or  society  of 
animals  A  in  the 

lowest  Stratum,  a       FlG'  309.  — Section  of  strata  containing  fossils  of 
r    1^1       J-rc  different  ages. 

slightly  different 

fauna  B  in  the  next  higher  stratum,  and  so  on  for  each 
stratum.  In  studying  a  number  of  other  outcrops  in  the 
same  vicinity,  we  find  that  the  same  sequence  of  faunas  pre- 
vails in  all.  We  thus  have  a  standard  section  for  beds  A—E. 
Now  suppose  that  in  the  right-hand  hill,  we  find,  at  the  base, 
rocks  which  contain  fauna  D,  and  above  that  fauna  E, 
while  the  higher  layers  afford  new  faunas  unlike  any  seen  in 


304  HISTORICAL  GEOLOGY 

the  first  locality.  The  coincidence  of  the  lower  fossils  with 
those  of  D-E  of  the  earlier  sections  establishes  our  starting 
point,  and  if  the  beds  are  conformable,  so  that  we  know 
there  was  no  interruption  in  deposition,  we  may  now  designate 
the  upper  faunas  as  F,  G,  H,  and  I.  This  process  of  matching 
and  piecing  out  local  sections  has  been  carried  on  until  we 
have  a  nearly  complete  series  of  sections  extending  from  the 
present  sediments  back  to  rocks  of  very  ancient  times. 

The  time  value  of  unconformities.  —  Thus  far  we  have 
considered  only  conformable  strata,  those  which  were  laid 
down  one  upon  the  other  without  interruption  of  any  kind. 
It  often  happens  that  the  section  under  study  contains  an 
unconformity.  Without  fossils  this  unconformity  would  tell 
us  that  the  region  had  emerged  from  the  water  after  the 
deposition  of  the  lower  beds,  had  been  eroded,  and  again 
submerged  before  the  upper  layers  were  formed;  but  we 
should  not  know  how  much  time  had  elapsed  while  it  re- 
mained as  land.  Suppose,  however,  that  the  beds  contain 
fossils,  and  that  the  fauna  just  below  the  unconformity  is  like 
B  in  our  first  section,  while  the  one  next  above  is  identical 
with  G.  We  then  know  that  faunas  C-F  are  lacking  and 
that  the  region  may  have  been  land  through  all  of  the  time 
which  was  required  for  the  gradual  evolution  of  fauna  B 
into  the  new  fauna  G}  or  that  it  was  land  during  let  us 
say  the  E  and  F  periods  and  that  the  deposits  of  the  C  and  D 
periods  were  worn  entirely  away  at  that  place. 

The  geologic  column.  —  The  completed  series  of  sections, 
often  called  the  geologic  column,  is  useful  as  a  standard  to 
which  we  may  refer  any  isolated  rock  formation  in  which  we 
can  find  the  necessary  fossils.  Furthermore,  an  examination 
of  it  showed  very  early  that  it  contained  certain  natural  divi- 
sions which  could  be  recognized  all  over  the  world,  and  from 
time  to  time  geologists  have  given  to  these  divisions  names 
which  are  in  general  use. 

The  dividing  lines  between  the  parts  were  usually  the  planes 
at  which  sudden  and  marked  changes  in  the  fossils  were 


HISTORY  OF  THE   EARTH  305 

found.  For  example,  fossil  reptiles  were  abundant  in  the 
rocks  which  represent  a  certain  part  of  the  geologic  column 
(Mesozoic  era),  but  above  that  section  they  are  rare,  while  on 
the  other  hand  the  mammals  appear  in  profusion.  This  point 
of  change  made  a  convenient  place  to  separate  one  division 
from  another.  At  first  such  changes  in  the  fossils  were  made 
the  sole  basis  for  subdividing  the  column,  but  now  we  are 
coming  to  realize  that  these  very  transformations  among  the 
animals  and  plants  are  brought  about  by  corresponding 
changes  in  the  conditions  under  which  they  lived ;  or,  in  other 
words,  by  changes  in  the  climate,  the  topography,  the  relations 
of  land  and  sea,  etc.  So,  we  now  try  to  divide  the  geologic 
record  at  the  points  where  these  revolutionary  physical  changes 
are  indicated,  and  to  make  corresponding  divisions  of  geologic 
time  itself.  Thus  there  are  two  kinds  of  divisions ;  one  for  the 
rocks  themselves,  and  the  other  for  the  time  represented  by 

the  rocks:  — 

i 

Time  Divisions  Rock  Divisions 

Era  Group 

Period  System 

Epoch  Series 

There  is  some  difference  of  usage  among  geologists  as  to 
how  the  geologic  column  should  be  divided,  particularly  as 
to  the  rank  of  certain  of  the  divisions.  The  classification 
here  adopted,  although  not  to  be  regarded  as  permanent, 
agrees  well  with  the  facts  now  known.  As  the  names  of  the 
divisions  will  be  constantly  used  in  later  pages,  the  following 
table  should  be  learned  thoroughly.1 

1  The  names  of  these  time  divisions  have  grown  up  in  a  somewhat  haphaz- 
ard way  in  the  course  of  a  century  or  more  of  progress  in  the  study  of 
geology.  At  an  early  time  it  was  supposed  that  certain  periods  were  dis- 
tinguished by  the  formation  of  particular  rocks,  and  so  we  have  such  names 
as  "Cretaceous"  for  the  period  when  the  chalk  (Latin  creta)  of  England  was 
produced.  Later  it  has  become  customary  to  name  periods  after  regions 
in  which  the  rocks  of  that  age  are  well  known  ;  thus  Devonian  is  named 
for  the  county  of  Devon  in  England. 


306  HISTORICAL  GEOLOGY 


Table  of  Geologic  Divisions 

Quaternary  period  and  system. 
Cenozo,c  era  and  group  \  Tertiary  period  and  system. 

Cretaceous  period  and  system. 
Comanchean  period  and  system. 


Mesozoic  era  and  group 


Paleozoic  era  and  group 


Jurassic  period  and  system. 
Triassic  period  and  system. 
Permian  period  and  system. 
Pennsylvanian  period  and  system. 
Mississippian  period  and  system. 
Devonian  period  and  system. 


Silurian  period  and  system. 
Ordovician  period  and  system. 
Cambrian  period  and  system, 
f  Keweenawan  l  period  and  system. 
Proterozoic  era  and  group  j  Ariimikean  1  period  and  system. 

I  Huronian  1  period  and  system. 
Archseozoic  era  and  group    Archaean  period  and  system. 

Periods  older  than  the  geologic  record.  —  Back  of  these 
periods  and  eras  there  stretches  a  vast  lapse  of  time  of  which 
we  know  so  little  that  the  use  of  definite  time  divisions  is 
hardly  justified  as  yet.  It  includes  the  beginning  of  the  earth 
and  its  development  through  a  series  of  exceedingly  slow 
changes. 

Imperfect  record  of  the  earth's  history.  —  If  we  had  as 
complete  a  knowledge  of  the  earth  in  its  earlier  periods  as  we 
have  of  its  present  state,  we  should  be  able  to  construct  a 
fairly  complete  history  of  it.  That  would  include  the  changes 
which  have  occurred  in  the  shapes  of  seas  and  land,  in  the 
mountains,  plains,  and  other  topographic  features,  in  the  dis- 
tribution of  volcanic  districts,  the  development  of  the  many 
groups  of  animals  and  plants,  the  fluctuations  of  climate,  and 
many  other  important  things.  As  it  is,  only  a  part  of  these 
facts  have  been  recorded  in  the  rocks;  only  a  small  portion 

1  These  names  arefcapplied  only  in  the  Lake  Superior  region  of  the  United 
States.  The  rocks  which  represent  the  oldest  periods  are  almost  devoid  of 
fossils,  and  it  is  therefore  hardly  possible  to  extend  the  same  names  to 
other  regions,  as  has  been  done  in  the  case  of  later  periods. 


HISTORY  OF  THE   EARTH 


307 


of  the  record  itself  is  now  exposed  and  accessible ;  and  much  of 
that  portion  still  awaits  investigation.  It  is  clear,  then,  that 
the  story  of  the  earth  can  be  little  more  than  outlined,  as  yet, 
and  that  it  grows  more  obscure  as  we  trace  it  back  into  the 

remote  past. 

QUESTIONS 

1.  What  is  suggested  by  the  finding  of  sea  shells        ^ 

in  a  series  of  horizontal  beds  on  top  of  a  mountain   n^T  x  I  *  *  * 
3000  feet  high,  as  in  Figure  310 ?  *' '  *  *  "  "  "  ' 

2.  What  changes  may  be  inferred  from  the  sec- 
tions of  sedimentary  rocks  represented  in  Figures  311,  312,  and  313  ? 


FIG.  311.  —  Sandstone  and 
conglomerate. 


FIG.  312.  —  Limestone 
and  shale. 


FIG.  313. —  Conglomer- 
ate and  sandstone  rest- 
ing on  folded  schist. 

3.  As  between  the  fishes  and  the  mammals,  which  would  you 
expect  to  find  preserved  in  the  older  rocks  ?     Why  ? 

4.  What  would  be  indicated  by  finding  a  comparatively  recent 
group  of  fossils  in  beds  lying  beneath  rocks  which  contained  much 
older  types  ? 

5.  What  kinds  of  fossils  would  you  expect  to  find  in  sediments 
deposited  in  river  valleys,  as  against  those  laid  down  in  the  open  sea  ? 

6.  In  what  kind  of  volcanic  rocks  might  fossils  be  found  ?     Why 
not  in  the  others  ? 

REFERENCES  ON  HISTORICAL  GEOLOGY 

CHAMBERLIN  and  SALISBURY  :   Geology,  Vols.  II,  III.     (New  York, 

1906.) 

An  excellent  and  elaborate  account  of  the  history  of  the  earth. 
JORDAN  and  HEATH  :    Animal  Forms.     (New  York,  1902.) 

An  easily  understood  description  of  the  more  important  types 
of  animals  as  they  are  to-day. 
JORDAN  and  KELLOGG  :    Evolution  and  Animal  Life.     (New  York, 

1907.) 
LUCAS  :    Animals  of  the  Past.     (New  York,  1902.) 

An  interesting  explanation  of  the  preservation  of  animals  as 
fossils,  with  a  description  of  certain  groups  which  are  now  extinct. 
SCOTT:  An  Introduction  to  Geology.     2d   ed.     (New  York,  1907.) 


CHAPTER  X 
ORIGIN  AND  DEVELOPMENT  OF  THE  EARTH 

The  earth  and  the  planets.  —  It  is  a  matter  of  common 
knowledge  that  the  earth  is  a  ball  which  revolves  about  the 
sun,  —  another  but  much  larger  body  of  similar  shape. 
Seven  other  planets  more  or  less  like  the  earth  also  wheel 
about  the  central  sun  (Fig.  314).  Among  them  are  the 
familiar  stars,  Jupiter  and  Venus.  Four  of  the  planets  are 
larger  than  the  earth,  three  are  smaller.  Two  are  nearer  the 
sun,  while  five  are  farther  away  from  it.  The  moon  is  a  much 


FIG.  314.  —  Diagram  of  the  solar  system.  The  size  of  the  sun  and  particu- 
larly the  planets  is  enormously  exaggerated  as  compared  to  the  breadth 
of  the  orbits. 

smaller  body,  which  is  controlled  by  the  gravity  of  the 
earth  and  therefore  revolves  about  it.  Most  of  the  other 
planets  have  similar  moons  (satellites),  —  Saturn  has  nine  of 
them.  In  addition  to  its  motion  around  the  sun,  each  of  the 
planets  rotates  on  its  own  axis,  much  as  a  top  spins  while 
gliding  about  on  the  floor.  It  is  a  significant  fact  that  all  of 
the  planets  move  in  the  same  general  direction,  and  the  nearly 
circular  paths  which  they  follow,  year  after  year,  lie  so  nearly 
in  the  same  plane  that  the  whole  system  may  be  compared 
in  shape  to  a  disk.  As  we  now  know  it,  this  solar  system 

308 


ORIGIN  AND  DEVELOPMENT  OF  THE  EARTH      309 

is  a  beautifully  adjusted  and  harmonious  family  of  planets 
which  we  may  be  sure  has  not  been  seriously  disturbed  during 
many  millions  of  years  in  the  past. 

THEORIES  OF  ORIGIN 

The  heavenly  bodies  have  always  interested  men  to  such 
a  degree  that  many  attempts  have  been  made  to  explain  their 
origin  and  arrangement.  Up  to  the  sixteenth  century  it  was 
believed  that  the  earth  was  the  central  and  largest  part  of  the 
universe,  and  that  the  sun,  moon,  and  stars  moved  about  it. 
Men  were  very  generally  of  the  opinion  that  these  bodies 
were  created  in  their  present  state  and  were  maintained  for 
the  express  purpose  of  giving  light  to  the  human  inhabitants 
of  the  earth.  It  was  only  after  Copernicus,  in  the  sixteenth 
century,  showed  that  the  earth  is  merely  a  small  member  of 
a  great  system,  that  this  narrow  and  egotistical  view  began  to 
give  way  to  a  broader  conception  of  the  heavens. 

The  nebular  theory  of  Laplace.  —  Near  the  end  of  the 
eighteenth  century  the  distinguished  French  mathematician 
Laplace  worked  out  a  most  ingenious  theory  which  seemed 
for  a  time  to  account  for  nearly  every  peculiarity  of  the  solar 
system.  He  suggested  that  it  was  originally  derived  from  a 
huge  spheroidal  mass  of  gas,  which  was  so  hot  that  even  the 
metals  and  the  materials  which  form  the  rocks  of  to-day  were 
then  expanded  into  an  incandescent  vapor  many  times  thin- 
ner than  air.  As  this  nebula  gradually  cooled,  it  shrank, 
and  its  shrinkage  made  it  rotate  faster  upon  its  axis.  The 
faster  it  whirled  the  stronger  became  the  centrifugal  force  on 
the  outside  of  the  spheroid.  This  tendency  of  things  to  fly 
off  into  space  was  at  first  counteracted  by  the  attraction  of 
gravity  within ;  but  eventually  it  became  equal  to  gravity  on 
the  equator  of  the  spheroid  and  the  material  there  then  ceased 
to  contract. 

The  equatorial  portion  was  then  left  as  a  ring  encircling 
the  ever  shrinking  remainder.  This  ring  is  supposed  to  have 


310  HISTORICAL  GEOLOGY 

condensed  into  a  ball,  which  became  the  outermost  planet 
(Neptune).  Other  rings  were  produced  one  by  one  in  the 
same  way,  until  all  the  planets  had  come  into  existence,  and 
the  remainder  of  the  original  mass  was  left  as  the  sun.  The 
satellites  of  the  planets  were  thought  to  have  originated  from 
equatorial  rings  left  behind  by  the  contracting  planets  be- 
fore they  solidified.  On  this  theory,  then,  the  earth  began  as 
a  globe  of  intensely  hot  vapor  which  cooled  down  to  a  liquid, 
and  finally  crusted  over  with  a  solid  shell  of  rock,  while  the 
interior  remained  in  the  molten  state.  At  that  stage  the  sur- 
face of  the  earth  was  so  hot  that  water  could  exist  on  it  only 
in  the  form  of  vapor.  The  atmosphere  was  then,  according 
to  the  theory,  very  heavy,  hot,  and  utterly  unfit  for  living 
things.  In  time  it  cooled  sufficiently  to  allow  the  water  vapor 
to  condense  and  fall  as  rain.  For  a  long  time  the  surface  was 
still  so  hot  as  to  boil  off  the  water  as  it  fell.  This  process 
gradually  cooled  the  surface,  however,  and  finally  its  tem- 
perature was  sufficiently  low  so  that  the  falling  rain  remained 
as  surface  water.  Then  the  oceans  began,  and  as  the  tem- 
perature of  the  water  was  reduced  and  the  atmosphere  be- 
came somewhat  freed  from  carbon  dioxide  and  other  unwhole- 
some gases,  the  surface  of  the  globe  became  fit  for  living 
things. 

For  a  century  or  more  Laplace's  theory  seemed  to  fit  the 
facts  then  known,  and  it  was  regarded  as  essentially  true. 
But  we  must  remember  that,  from  the  very  nature  of  the 
problem,  it  is  unlikely  that  any  theory  of  the  earth's  origin 
can  be  proved.  The  more  critical  studies  of  recent  years 
have  shown  that  the  Laplacian  hypothesis  presents  many 
serious  difficulties.  Efforts  have  been  made  to  meet  these  by 
various  changes  in  the  details  of  the  theory,  but  the  more 
serious  objections  were  not  removed  by  any  of  these  modi- 
fications, and  doubt  arose  even  as  to  the  fundamental  ideas 
of  the  hypothesis.  For  example,  it  now  seems  improbable 
that  the  materials  which  became  the  planets  could  have 
separated  from  the  equatorial  portion  of  the  nebula  in  the 


ORIGIN  AND   DEVELOPMENT  OF  THE  EARTH      311 

form  of  rings ;  and  many  other  difficulties  of  an  even  more 
fundamental  nature  have  been  pointed  out.  A  new  theory, 
which  seems  to  explain  the  facts  we  now  have  at  command 
better  than  did  the  older  hypotheses,  has  recently  been  worked 
out  on  a  quite  different  basis.  It  is  known  as  the  Planetesi- 
mal  theory.1 

The  planetesimal  theory.  —  Many  more  nebulae  are  known 
now  than  were  known  when  Laplace  advanced  his  theory. 
But  among  them  all  none  have  been  found  which  have  a  cen- 
tral mass  surrounded  by  a  ring.  On  the  other  hand,  re- 
searches show  that  the  great  majority  of  nebulae  are  spiral 
in  form  (Figs.  315  and  316).  Such  a  nebula  consists  of  a 
luminous  center  with  spiral  arms  or  streamers  issuing  from 
opposite  sides.  On  these  arms  there  are  knotlike  condensa- 
tions at  various  points.  The  spiral  form  suggests  that  the 
whole  mass  has  a  whirling  motion  about  the  center.  It  is 
thought  that  the  thinner  portions  of  these  nebulae  are  com- 
posed of  scattered  particles  of  various  sizes,  while  in  the  knots 
the  particles  are  less  scattered.  All  of  the  particles,  including 
those  of  the  knots,  are  believed  to  be  revolving,  each  in  its 
own  independent  orbit,  about  the  central  mass,  much  as  the 
earth  revolves  about  the  sun.  Since  these  bodies  behave  like 
tiny  planets,  they  are  called  planetesimals.  The  suggestion 
is  made  that  our  solar  system  may  have  grown  from  such  a 
nebula.  It  is  conceived  that  the  knotlike  bodies  in  the  arms 
became  the  centers  of  growth  for  the  planets,  gradually  gather- 
ing in  the  planetesimals  about  them.  Some  of  the  "  shooting 
stars  "  or  meteorites  that  enter  our  atmosphere  are  probably 
but  planetesimals  still  being  gathered  in,  —  a  suggestion  that 
the  growth  of  the  earth  is  still  continuing,  although  with 
exceeding  slowness. 

On  the  new  hypothesis,  then,  the  earth  grew  from  a  nebular 
knot  to  its  present  size  by  the  slow  ingathering  of  the  smaller 
particles  or  planetesimals.  In  the  earlier  stages  of  its  existence 

1  The  work  of  Professor  T.  C.  Chamberlin  of  the  University  of  Chicago, 
assisted  on  the  mathematical  side  by  Dr.  F.  R.  Moulton. 


312  HISTORICAL  GEOLOGY 

as  a  planet,  it  may  have  had  no  atmosphere.  If  that  was  true, 
the  great  processes  of  erosion  could  not  then  have  been  in 
operation,  since  they  are  largely  dependent  upon  the  pres- 
ence of  an  atmosphere. 


FIG.    315.  —  Spiral    nebula.     (Photograph    by    Ritchey   at    the    Ycrkes 
Observatory.) 

Gases,  and  therefore  air,  have  a  marked  tendency  to  expand 
and  diffuse  themselves  through  space.  It  is  known  that  the 
earth  is  able  to  keep  its  atmosphere  only  because  of  its  strong 
gravity,  and  that  a  few  gases,  notably  free  hydrogen,  are  too 
active  to  be  held  by  it.  Since  in  small  bodies  gravity  is  much 


ORIGIN  AND  DEVELOPMENT  OF  THE   EARTH      313 

weaker  than  in  large  ones,  it  is  clear  that  small  bodies  are  less 
likely  to  hold  an  atmosphere  than  large  ones.  Thus  the  moon 
seems  to  be  quite  devoid  of  an  atmosphere  to-day.  If  the 
earth  had  gravity  enough  to  hold  an  atmosphere  at  the  outset, 


FIG.    316.  —  Spiral    nebula.     (Photograph    by    Ritchey   at   the    Yerkes 
Observatory.) 

it  probably  had  one.  If  its  gravity  was  not  sufficient  at  first, 
it  eventually  became  so,  as  the  earth  grew ;  and  so  the  planet 
slowly  acquired  an  atmosphere  and,  later,  oceans.  The 
method  by  which  the  atmospheric  gases  were  supplied  is 
simple,  and  doubtless  continued  in  a  measure  down  to  the 


314  HISTORICAL  GEOLOGY 

present  day.  The  overpowering  force  of  gravity  tends  to 
compress  the  material  of  the  earth  into  an  ever  denser  form, 
and  this  compression  generates  heat.  As  the  planet  grew 
larger,  its  gravity  increased  in  strength.  The  interior  was 
therefore  more  compressed  and  became  constantly  hotter. 
All  rocks,  whether  such  as  belong  to  the  interior  of  the  earth, 
or  such  as  come  down  as  meteorites,  contain  a  variety  of  gases 
or  material  out  of  which  gases  may  be  formed  by  heat.  The 
growing  pressure  and  heat  of  the  earth's  interior  are  therefore 
supposed  gradually  to  have  driven  out  some  of  these  gases, 
and  thus  furnished  the  material  for  an  atmosphere,.  The 
gases  issuing  from  volcanoes  to-day  illustrate  this  process. 
So,  also,  particles  of  gas  are  supposed  to  have  been  parts  of  the 
nebula  and  to  have  been  gathered  in  from  the  outside.  It 
was  therefore  only  necessary  for  the  earth  to  acquire  suffi- 
cient force  of  gravity  to  hold  these  gases  upon  its  surface,  to 
enable  it  to  accumulate  an  atmosphere.  At  first  this  prob- 
ably consisted  of  such  gases  as  carbon  dioxide,  nitrogen,  and 
water  vapor,  as  these  are  the  ones  chiefly  given  off  by  rocks 
and  meteorites.  Carbon  dioxide,  however,  is  chemically 
active  and  must  have  been  partly  disposed  of  by  uniting  with 
the  rocks,  while  nitrogen  is  little  disposed  to  enter  into 
combination  and  must  have  constantly  accumulated.  At 
the  same  time  certain  agencies  that  decompose  water  vapor 
probably  gave  rise  to  oxygen. 

As  the  water  vapor  accumulated  it  eventually  condensed 
into  rain,  and  with  the  rain  came  the  beginning  of  other 
geological  processes.  At  first,  the  water  circulating  through 
the  porous  outer  portion  of  the  earth  dissolved  out  minerals 
in  certain  places  and  deposited  them  elsewhere  in  the  form  of 
cementing  material  and  vein  fillings.  Later,  as  the  ground 
water  level  rose,  ponds  and  lakes  appeared  in  the  lowest  de- 
pressions of  the  earth's  surface,  for  the  haphazard  infall  of  the 
solid  planetesimals  probably  left  many  irregularities,  and  in 
addition  there  were  numerous  volcanic  craters.  As  time 
went  on  these  lakes  grew  and  coalesced  into  great  seas  and 


ORIGIN  AND  DEVELOPMENT  OF  THE  EARTH      315 

oceans,  and  thus  the  hydrosphere  became  fully  developed. 
At  the  same  time  the  erosion  of  the  uplands  commenced  and 
sediments  began  to  accumulate  in  the  depressions. 

With  the  establishment  of  these  great  processes,  suitable 
heat,  light,  moisture,  air,  and  all  the  other  conditions  which 
seem  necessary  for  the  existence  of  life  were  present,  and  life 
probably  began.  But  the  origin  of  the  first  living  things  is 
still  among  the  unsolved  problems  of  science.  There  is, 
however,  every  reason  to  believe  that,  whatever  their  origin, 
the  earliest  forms  of  life  were  very  simple,  and  probably  more 
like  the  lowest  plants  of  to-day  than  like  animals.  From 
these  early  forms  all  later  kinds  are  thought  to  have  been 
derived  by  a  vast  number  of  slow  changes,  probably  occupying 
many  tens  of  millions  of  years. 

Volcanoes  probably  appeared  on  the  earth  at  a  compara- 
tively early  stage  in  its  history,  long  before  it  had  grown  to  the 
size  of  the  moon.  There  is  reason  to  believe  that  volcanic 
action  gradually  increased  in  prominence  and  reached  its 
climax  after  the  earth  had  attained  its  present  size. 

The  two  theories  compared.  —  In  many  respects  these 
two  theories  of  the  earth's  origin  are  directly  opposed  to  each 
other.  Under  the  Laplacian  view  the  planet  was  at  first 
larger  and  hotter  than  now,  and  it  continually  cooled  and  con- 
tracted until  it  became  partly  or  entirely  solid.  Under  the 
planetesimal  theory  the  earth  grew  larger  by  gathering  in 
material  from  outside,  and  it  was  not  necessarily  ever  hotter 
than  now,  if  as  hot.  Under  the  first,  the  atmosphere  has 
become  thinner  and  poorer,  from  the  time  when  it  was  exceed- 
ingly heavy,  dense,  and  composed  largely  of  steam ;  under 
the  second  it  has  grown  larger  and  richer  in  material,  and 
it  was  never  hot.  According  to  the  Laplacian  theory  the 
oldest  rocks  which  we  might  hope  to  find  would  be  entirely 
igneous,  —  portions  of  the  original  crust  which  coated  the 
surface  of  the  molten  globe ;  but  under  the  later  hypothesis, 
we  should  expect  not  only  the  igneous  materials  derived  from 
volcanic  eruptions,  but  sedimentary  deposits  as  well,  in  the 


316  HISTORICAL  GEOLOGY 

very  oldest  rocks  we  could  hope  to  reach.  For  still  another 
contrast,  we  may  turn  to  the  future  fate  of  the  earth  as 
forecast  by  each  of  the  opposing  theories.  If  we  follow  the 
older  conception,  we  must  predict  a  gradual  cooling  and  final 
refrigeration  of  the  earth  and  even  the  sun,  the  absorption  of 
its  dwindling  atmosphere  within  itself,  and  the  death  of  all 
living  creatures.  Under  the  newer  hypothesis,  the  outlook  is 
less  gloomy.  Looking  ahead  as  far  as  imagination  will  safely 
carry  us,  we  can  see  no  prospect  of  destructive  changes.  The 
infinitesimally  slow  growth  of  the  earth  and  its  atmospheric 
envelope  should  continue,  and  the  evolution  of  living  things 
into  higher  and  better  kinds  should  have  ample  time  for 
accomplishment. 

QUESTIONS 

1.  What  prevents  the  planets  from  leaving  the  sun? 

2.  If  Jupiter  should  pass  close  to  the  earth,  what  would  happen 
to  the  moon  ?     Why  ? 

3.  If  the  Laplacian  theory  is  correct,  what  materials  should  have 
formed  the  first  solid  part  of  the  earth  ?      In  what  order  should 
other  materials  have  been  added  ? 

4.  If  the  planetesimal  theory  is  correct,  which  would  be  brought 
into  operation  first,  weathering  or  wave  action  ? 

5.  On  the  same  hypothesis  would  the  waters  of  the  first  seas 
have  been  fresher  or  salter  than  in  existing  oceans?     Under  the 
Laplacian  hypothesis  ? 

6.  On  the  Laplacian  theory  how  should  the  volcanic  activity  of 
early  times  compare  with  that  of  the  present? 

7.  One  of  the  satellites  of  Mars  revolves  about  the  planet  much 
faster  than  Mars  turns  on  its  axis.     How  does  this  fact  bear  on  the 
Laplacian  theory  ? 

REFERENCES 

COMSTOCK  :    A  Textbook  of  Astronomy.     (New  York,  1903.) 

MOULTON  :    An  Introduction  to  Astronomy.     (New  York,  1906.) 

POOR  :   The  Solar  System.     (New  York,  1908.) 

TODD  :   A  New  Astronomy.     (New  York,  1897.) 

YOUNG  :   A  Textbook  of  General  Astronomy.     (Boston,  1893.) 


CHAPTER  XI 
THE   ARCHEOZOIC   ERA 

The  oldest  rocks.  —  The  most  ancient  part  of  the  earth's 
history  of  which  we  have  any  tangible  record  in  the  rocks  is 
quite  as  shadowy  and  obscure  as  the  dim  legendary  period  of 
human  annals.  The  record  has  been  in  large  measure  de- 
stroyed, and  that  which  remains  is  confused  and  difficult  to 
interpret.  The  rocks  which  were  made  during  the  Archse- 
ozoic  era  are  the  oldest  of  which  we  have  any  knowledge. 
They  compose  what  is  called  the  Archcean  system.  This  un- 
derlies all  later  systems  of  rocks,  and  over  most  of  the  globe 
it  is  still  buried  beneath  thick  layers  of  younger  rocks  (Fig. 
317).  In  most  places  where  Archa3an  rocks  can  now  be 


FIG.  317.  —  Ideal  cross  section  of  North  America,  showing  how  the  Archaean 
rocks  underlie  all  others. 

studied  they  have  been  uncovered  by  erosion.  Such  regions 
may  have  been  more  deeply  eroded  than  others  because  they 
have  been  repeatedly  uplifted.  In  the  eastern  half  of  Canada 
the  ArchaBan  rocks  reach  the  surface  over  a  large  area,  inter- 
rupted only  by  bands  or  patches  of  the  younger  rocks.  They 
are  exposed  again  in  a  long  strip  between  the  Appalachian 
Mountains  and  the  Coastal  Plain,  stretching  from  Alabama 
to  New  England,  in  the  cores  of  certain  anticlines  among  the 
mountains  of  western  United  States,  and  in  other  situations. 
In  other  countries  similar  exposures  of  Archa3an  rock  have  been 
found,  particularly  in  Brazil,  central  Africa,  and  Scandinavia. 
B.  &  B.  GEOL.  — 18  317 


318 


HISTORICAL  GEOLOGY 


The  downward  limit  of  the  Archaean  rocks  is  unknown ; 
probably  it  will  never  be  attained.  The  upper  limit  is  the 
surface  which  separates  it  from  the  Proterozoic  group.1 

Complexity  of  the  Archaean.  —  The  study  of  the  Archaean 
rocks  shows  that  they  have  been  profoundly  disturbed  and 
their  original  form  greatly  altered.  They  have  been  folded, 
crumpled,  and  contorted  in  the  most  intricate  way.  They 
are  broken  by  faults  and  interrupted  by  masses  of  igneous 
rocks  which  have  been  intruded  into  them.  Great  batho- 

liths  of  granite 
are  so  common 
in  them  as  to  be 
almost  character- 
istic of  the  Ar- 
chaean system. 
Study  of  the  in- 
trusions reveals 
the  fact  that 
some  have  broken  through  others,  showing  that  they  are  of 
many  different  ages  (Fig.  318).  Many  of  these  structures 
have  been  folded,  and  broken,  since  the  volcanic  activity 
ceased. 

The  known  Archaean  rocks  have  been  greatly  changed.  — 
In  an  earlier  chapter  (pp.  67,  77)  it  has  been  said  that  un- 
der the  tremendous  weight  of  miles  of  overlying  rock  even  the 
strongest  materials  may  be  crushed,  squeezed  out  into  thin 
plates,  and  crumpled  like  leaves  of  paper.  At  the  same  time 
certain  portions  of  the  rocks  are  dissolved  bit  by  bit  and 
rearranged  into  new  minerals  which  are  better  suited  to  the 
conditions  of  high  pressure  and  temperature  of  the  depths. 
Thus,  black  basalts  become  glistening  green  schists,  granites 
may  become  gneisses,  limestone  crystallizes  in  the  form  of 
marble,  and  shales  are  transformed  into  schists  spangled  with 
flakes  of  mica. 


FIG.   318.  —  Successive   intrusions  of   igneous   rock. 
How  do  the  intrusions  rank  in  order  of  age  ? 


1  By  some  geologists  Archaean  is  made  to  include  all  the  rocks  below  the 
Cambrian,  but  this  usage  is  not  common  to-day. 


THE  ARCHEOZOIC  ERA  319 

Almost  all  the  Archaean  rocks  have  been  buried  to  great 
depths  and  subsequently  uncovered  by  erosion.  The  Archaean 
rocks  are  therefore  commonly  schists  and  gneisses.  Certain 
portions  of  later  systems  of  rock  are  likewise  schistose,  but 
in  the  Archaean  alone  are  the  schists  and  gneisses  almost 
universal  (Fig.  319). 

Metamorphic  rocks  are  of  course  always  derived  from  other 
rocks  of  igneous  or  sedimentary  origin  (p.  41).  It  is  often 
impossible  to  determine  the  original  character  of  some  of  the 
Archaean  rocks,  while  that  of  others  may  be  discovered  by 
study.  It  is  a  singular  fact  that  among  the  oldest  rocks 
which  have  yet  been  found  in  the  Archaean  system  are  green 
schists  which  were  once  surface  volcanic  materials,  such  as 
lava  flows  and  cinders.  These  must  have  been  cast  out  during 
successive  eruptions  upon  a  still  older  surface,  but  as  yet  that 
surface  has  not  been  identified.  It  is  known  that  some  of 
the  gneisses  were  once  granites,  —  parts  of  batholiths  which 
were  intruded  into  these  green  schists,  and  subsequently 
metamorphosed. 

The  Archaean  system  is  not  composed  entirely  of  igneous 
rocks,  as  it  was  formerly  thought  to  be.  In  the  Lake  Superior 
region  it  contains  small  bodies  of  iron  ore,  metamorphosed 
conglomerate,  and  slate.  In  China  and  Finland  even  lime- 
stone has  been  found  in  the  oldest  rocks,  which  seem  to  be  of 
Archaeozoic  age. 

Conditions  in  Archaean  time.  —  Evidently  we  can  hope  to 
learn  but  little  of  Archaean  times  from  such  a  disordered  and 
obscure  record  as  this,  but  the  very  remoteness  of  that  era 
lends  interest  to  any  bit  of  information  about  it.  Nothing 
is  clearer  than  that  there  were  many  successive  volcanic 
eruptions  and  intrusions, — probably  more  than  have  occurred 
in  any  later  era.  That  the  weathering  and  erosion  of  the 
lands  were  already  in  progress  is  suggested  by  the  presence  of 
slates  and  other  sedimentary  rocks,  for  slates  were  once  clay 
and  clay  is  made  from  many  kinds  of  rocks  by  chemical  decay 
and  the  sorting  action  of  water. 


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THE  ARCHEOZOIC  ERA  321 

The  presence  of  limestone  may  mean  that  shell-bearing 
animals  were  already  in  existence,  but  the  fact  that  some 
limestones  are  even  to-day  formed  by  the  direct  precipitation 
of  lime  carbonate  from  water  leaves  the  question  in  doubt. 
Beds  of  graphite,  which  are  thought  to  be  simply  metamor- 
phosed coal  seams,  indicate  that  plants  lived  in  Archaean  time. 
No  fossils  have  been  found  in  the  Archaean  rocks  and  so  we 
know  nothing  of  the  real  character  of  the  life  which  may  have 
existed  then.  All  we  can  be  safe  in  imagining  about  the  plants 
and  animals  of  the  era  is  that  they  were  on  the  average  much 
simpler  and  lower  in  the  scale  of  life  than  those  which  exist 
to-day. 

Length  of  the  Archaeozoic  era.  —  It  is  difficult  to  imagine 
the  vast  length  of  time  embraced  in  the  Archaeozoic  era.  Since 
the  bottom  of  the  Archaean  rocks  has  never  been  reached  it 
is  clear  that  we  know  nothing  of  the  earlier  part  of  that  era. 
Yet  even  the  knowable  part  gives  us  a  fragmentary  story  of 
many  successive  volcanic  disturbances  and  several  distinct 
periods  of  folding  and  metamorphism.  From  the  clearer 
record  of  later  times  we  know  that  such  changes  take  place 
slowly  and  are  separated  by  periods  of  quiet,  often  to  be 
measured  in  millions  or  tens  of  millions  of  years.  Considera- 
tions such  as  these  have  led  to  the  conjecture  that  the  Ar- 
chaeozoic era  may  have  been  longer  than  all  the  later  periods 
together. 

QUESTIONS 

1.  In  some  places  the  Archaean  system  is  found  to  contain  both 
gneiss    (once   granite)    and    schistose   basalt.     Which   of    the    two 
would  you  consider  the  older  (1)  if  the  gneiss  contained  rough  frag- 
ments of  the  basalt  within  itself  and  if  the  main  mass  of  basalt  were 
cut  by  branching  layers  of  gneiss,  or  (2)  if  the  gneiss  were  crossed 
by  layers  of  basalt  continuous  with  the  main  mass  of  the  latter  ? 

2.  Why  should  we  not  expect  to  find  fossils  in  schist  even  though 
the  original  mud  from  which  it  was  derived  was  filled  with  shells  ? 

3.  An  old  name  for  the  Archaean  system  is  "Basement  Complex." 
Why  is  this  a  good  descriptive  phrase  ? 


CHAPTER  XII 


THE   PROTEROZOIC    ERA 

What  it  represents.  —  The  oldest  rocks  which  contain 
numerous  fossils  are  those  of  the  Paleozoic  group.  Between 
these  fossil-bearing  sedimentary  rocks  of  the  Paleozoic  and 
the  intricate  complex  which  records  Archeozoic  time,  there  is 
in  many  places  a  thick  group  of  systems,  partly  sedimentary 
and  partly  igneous  in  origin,  which  represents  a  vast  lapse  of 
time  between  these  two  eras.  This  time  is  the  Proterozoic 1 
era  (often  called  also  the  Algonkian  period). 

PROTEROZOIC  ROCKS  OF  THE  LAKE  SUPERIOR  REGION 

The  Proterozoic  rocks  are  nowhere  better  known  than  in 
the  vicinity  of  Lake  Superior.  In  certain  parts  of  this  region 

the  entire  group  is  divided 
into  four  systems  which  are 
separated  from  each  other 
by  unconformities.  In  other 
localities  only  three  or  two 
divisions  are  distinguished. 
Where  four  systems  are 
known,  they  are  called  lower 
and  upper  Huronian,  Animi- 
kean,  and  Keweenawan. 

Basal  unconformity. — 
The  basal  formation  may  in- 
clude a  conglomerate  which 
contains  rounded  pebbles  of 

at  a    locality    in    Michigan.      (After     Schist     and     gneiss    derived 

V w^r-   A  *  A  from  the  Archaean  rocks  be- 

Wmch  is  the  younger  of  the  two 
formations,  and  what  is  the  evidence  ?     neath  (Fig.  320) .     No  better 

1  From  two  Greek  words  meaning  "earlier  life." 
322 


THE   PROTEROZOIC  ERA 


323 


FIG.  321.  —  Stereogram  showing  down-folded  rem- 
nants of  Proterozoic  rocks  surrounded  by  the 
Archaeozoic  complex. 


proof  could  be  desired  that  the  Archaean  rocks  had  been  folded, 
metamorphosed,  laid  bare  as  land,  and  profoundly  eroded 
before  the  Proterozoic  rocks  were  deposited  upon  them. 

Huronian  system.  —  The  Huronian  rocks  are  quartzite, 
limestone,  and  slate,  with  the  addition  of  beds  of  iron  ore  and 
jasper.  Where  metamorphosed  the  predominating  rocks  are 
schists.  The  beds 
are  usually  much 
folded  and  they 
are  exposed  at  the 
surface  as  long 
down-folded  bands 
within  the  out- 
crops of  the  Ar- 
chaean (Fig.  321). 
In  at  least  one  dis- 
trict a  well-marked 
unconformity  di- 
vides the  Huronian  strata  into  two  systems,  the  lower  of  which 
is  evidently  much  older  than  the  upper. 

Igneous  intrusions  of  different  ages  cut  through  them  here 
and  there,  and  lava  flows  are  sometimes  found  interbedded 
with  the  sediments  themselves.  Around  the  batholiths  of 
granite  and  the  other  large  intrusions,  the  rocks  may  be  altered 
to  schists ;  and  it  then  becomes  difficult  to  discriminate  them 
from  those  other  schists  which  belong  to  the  Archaeozoic 
group. 

Animikean  system.  —  Unconformably  above  the  Huronian 
rests  the  Animikean,1  another  system  of  sedimentary  rocks 
and  lava  flows,  which  is  in  general  much  like  the  Huronian 
(Fig.  322).  On  the  average,  however,  the  rocks  are  less 
folded  and  less  metamorphosed,  —  in  some  places  not  at 
all.  The  quartzites  and  slates  are  traversed  by  a  few  dikes 
and  larger  intrusions  of  later  age.  Iron  ore  has  been  men- 

1  The  geologists  of  the  U.S.  Geological  Survey  class  the  Animikean  as 
Upper  Huronian. 


324 


HISTORICAL  GEOLOGY 


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tioned  as  a  constituent  of  the  Huronian.  In 
the  Animikean  the  largest  and  richest  deposits 
of  that  indispensable  ore  that  are  yet  known 
have  been  found  (Fig.  323).  They  occur  in 
the  form  of  thick  beds  in  the  sedimentary 
rocks.  Some  of  the  Animikean  formations 
originally  contained  a  large  amount  of  iron 
minerals,  together  with  quartz  and  other  im- 
purities ;  this  was  further  enriched  in  certain 
spots  where  the  underground  waters  dissolved 
out  everything  except  the  iron  minerals  and, 
in  some  cases,  even  rilled  the  pores  thus  left 
with  still  more  oxide  of  iron. 

The  mines  of  the  Lake  Superior  region 
supply  more  than  80  per  cent  of  the  ore 
from  which  is  derived  the  iron  used  in  the 
great  industries  of  this  country.  This  is 
equal  to  more  than  one  third  of  the  world's 
output.  More  ore  is  now  taken  from  a  single 
mine  in  the  Mesabi  district  of  Minnesota 
each  year  than  was  mined  in  the  entire 
United  States  before  the  Civil  War. 

Keweenawan  system. — Still  a  third  great 
system  lies  upon  the  eroded  edges  of  the 
Animikean  strata  and  occasionally  laps  over 
upon  even  older  formations.  In  this  we  have 
the  record  of  one  of  the  greatest  episodes  of 
local  volcanic  activity  known  in  geologic  time. 
The  eruptions  seem  not  to  have  come  from 
definite  craters,  but  the  fluid  lava  simply 
welled  up  through  cracks  in  the  surface  and 
spread  over  wide  areas.  A  series  of  these 
flows  accumulated  one  above  the  other  to 
a  depth  estimated  at  more  than  6  miles. 
The  great  number  of  the  flows  may  be  ap- 
preciated when  we  consider  that  most  of 


THE  PROTEROZOIC  ERA 


325 


them  were  less  than  one  hundred  feet  in  thickness.     Later  in 

this  period  the  eruptions  apparently  came  at  wider  intervals, 

and     meanwhile 

coarse   sandstones 

were  deposited  in 

the    same   region. 

Finally    the    lava 

ceased  to  flow  out 

and  so,  toward  the 

close  of  the  period, 

only    sedimentary 

rocks  were  made. 

These    lavas    and 

sandstones      form 

the    Keweenawan 

system.     Since 

they   were    laid 

down,   they    have 

been     moderately 

tilted    but    not 

much  altered. 

It  appears  that 
the  lava  originally 
contained  minute 
quantities  of  cop- 


FIG.  323.  —  Distribution  of  ore  deposits  in  the 
Proterozoic  rocks  of  eastern  United  States  ;  iron 
districts  are  shown  by  the  black  patches,  and 
copper  deposits  by  the  crosses. 

per.     Part  of  this 

copper,  furnished  in  solution  to  the  active  underground  waters, 
was  deposited  in  certain  porous  layers  in  the  sandstones  and 
gravels,  as  well  as  in  the  cindery  portions  of  the  lava  flows 
themselves.  From  these  enriched  bands  vast  quantities  of 
pure  copper  have  been  mined  during  the  last  few  decades. 


PROTEROZOIC  ROCKS  IN  OTHER  REGIONS 

Rocks  of  Proterozoic  age  are  found  in  many  parts  of  this 
and  other  continents,  but  the  formations  cannot  be  matched 


326 


HISTORICAL  GEOLOGY 


closely  with  those  of  the  Lake  Superior  region.     This  is  true 
chiefly  because  the  necessary  fossils  are  lacking. 

In  the  Grand  Canon  of  Arizona  Proterozoic  strata  are 
again  well  exposed,  but  they  are  unlike  the  Lake  Superior 
formations  in  details.  The  lower  walls  of  the  canon  reveal 
the  complex  schists  of  the  Archaean.  Upon  these  rests  un- 
conformably  a  tilted  pile  of.  sedimentary  strata  (Fig.  324). 


FIG.  324.  —  Ideal  cross  section  of  the  Grand  Canon  of  the  Colorado  River 

in  Arizona. 

In  spite  of  their  great  age  they  are  neither  folded  nor  notably 
metamorphosed.  These  in  turn  were  largely  removed  during 
a  still  later  period  of  erosion,  so  that  the  Cambrian  sandstone 
was  deposited  horizontally,  not  only  upon  the  beveled  edges 
of  the  Proterozoic  formations,  but  out  over  the  Archaean  also: 
Proterozoic  rocks  are  well  known  in  Scotland,  Sweden,  and 
China  (Fig.  325),  and  have  been  studied  in  considerable 


FIG.  325. — A  section  through  the  ancient  rocks  at  a  point  in  Northern 
China,  showing  the  Archaeozoic  rocks  (A),  overlain  by  a  thick  series  of 
folded  beds  of  Proterozoic  age  (B),  and  upon  both  resting  Proterozoic  lime- 
stone and  shale  (C),  much  less  folded.  The  Cambrian  rocks  (Z>)  rest  un- 
conformably  on  the  others. 

detail.  In  each  case  there  appear  to  be  two  or  more  systems 
between  the  Archaean  and  the  Cambrian,  separated  from 
each  by  a  pronounced  unconformity.  Where  there  are  two 
systems  the  older  is  usually  intensely  folded  and  metamor- 
phosed, although  still  plainly  made  up  of  sedimentary  rocks ; 


THE   PROTEROZOIC  ERA  327 

while  the  younger  consists  of  slates,  quartzites,  and  lime- 
stones which  are  neither  closely  folded  nor  much  altered. 

GENERAL  CHARACTERISTICS  OF  THE  PROTEROZOIC  GROUP 

Sedimentary  rocks  but  with  some  igneous.  —  Having 
learned  something  about  the  Proterozoic  systems  in  widely 
separated  regions,  we  may  proceed  to  consider  the  things 
which  are  characteristic  of  the  group  as  a  whole  and  of  the 
long  periods  of  time  during  which  it  was  being  formed.  In 
each  case  the  rocks  which  make  up  the  group  were  derived 
chiefly  from  ordinary  sediments.  They  were  once  gravel, 
sand,  clay,  and  ooze  spread  out  upon  the  sea  floor  or  upon  the 
low-lying  lands.  They  have  since  been  cemented  into  solid 
rocks;  they  have  been  folded,  mildly  in  some  places  and 
intensely  in  others;  and  some  of  them  have  been  metamor- 
phosed into  slates,  schists,  and  gneisses.  Many  kinds  of  lava 
have  been  forced  up  through  them  at  different  times.  These 
either  spread  out  on  the  surface  as  flows,  or  solidified  beneath 
in  the  form  of  dikes,  sills,  batholiths,  and  other  intrusions, 
which  interrupt  the  stratified  rocks  and  complicate  the  study 
of  the  structure.  As  would  be  expected,  the  older  Proterozoic 
formations  are  often  much  more  deformed  than  the  younger, 
because  they  have  passed  through  more  epochs  of  folding. 

The  ArchaBan  system,  we  learned,  includes  some  beds  of 
sedimentary  rock,  but  the  vast  body  of  that  ancient  mass  is 
of  either  igneous  or  doubtful  origin.  In  the  Proterozoic 
group,  on  the  other  hand,  the  proportions  are  reversed,  and 
the  sedimentary  strata  predominate  overwhelmingly. 

Unconformity  general  but  not  universal.  —  We  have  seen 
that  in  each  district  where  the  rocks  have  been  fully  studied 
the  Proterozoic  group  is  separated  from  the  Archeozoic  by  a 
great  unconformity.  This  clearly  shows  that  the  regions  had 
been  lands  cut  down  by  weathering  and  erosion  until  the  very 
roots  of  the  Archaean  mountains  were  laid  bare  and  planed 
off, — and  all  this  before  the  Proterozoic  sediments  began  to 


328  HISTORICAL  GEOLOGY 

be  deposited.  This  unconformity  evidently  tells  of  a  very 
long  lapse  of  time  between  the  deposition  of  the  Archaean 
and  Proterozoic  rocks,  —  a  time  otherwise  unrecorded  in  the 
rocks  which  we  know.  Because  of  this,  and  because  of  the 
wide  distribution  of  the  unconformity,  it  is  generally  regarded 
as  one  of  the  greatest  interruptions  in  the  geologic  record. 
But  no  unconformity,  however  widespread,  can  exist  all  over 
the  globe.  The  very  same  facts  which  indicate  that  the  lands 
were  deeply  eroded  prove  that  the  material  worn  off  was  as 
continually  being  deposited  elsewhere;  and  in  those  areas 
where  deposition  was  in  progress  no  unconformity  resulted. 
It  has  been  suggested  that  the  sediments  which  were  deposited 
then,  as  now,  in  the  deep  ocean  basins  have  never  been  raised 
into  land,  and  hence  are  still  unknown  to  us. 

Unconformities  within  the  Proterozoic  group.  —  Other 
notable  unconformities  serve  to  divide  the  Proterozoic  into 
two  or  more  systems.  In  Minnesota  the  Animikean  sand- 
stones and  shales  rest  at  a  moderate  inclination  upon  closely 
folded  slates  and  quartzites  of  the  Huronian.  Furthermore, 
some  of  the  dikes  in  the  Huronian  rocks  do  not  pass  up  into 
the  Animikean  system.  Such  an  unconformity  is  conspicuous 
when  it  can  be  seen  in  the  side  of  a  quarry  or  a  ravine.  No 
one  of  these  interruptions  in  the  strata  has,  however,  been 
traced  across  any  continent,  much  less  over  several  conti- 
nents; and  the  divisions  themselves,  therefore,  can  be  used 
only  in  the  region  where  they  are  known  to  apply.  Protero- 
zoic rocks  are  generally  separated  from  the  Cambrian  system, 
which  overlies  them,  by  another  great  unconformity,  a  de- 
scription of  which  will  be  found  in  the  next  Chapter. 

Duration  of  the  Proterozoic  era.  —  Just  as  the  remote 
ancient  periods  of  human  history  are  long  in  comparison  with 
the  subsequent  centuries,  so  the  Proterozoic  era  was  im- 
mensely long  as  compared  with  later  periods.  In  the  course  of 
a  century  only  a  few  feet  of  average  sediments  are  deposited, 
and  of  limestones  perhaps  not  even  one  foot.  Yet  the  Huro- 
nian sediments  of  Michigan  are  alone  said  to  be  more  than 


THE   PROTEROZOIC  ERA  329 

13,000  feet  thick,  while  the  Keweenawan  lava  flows  and  sand- 
stones may  have  a  thickness  of  35,000  feet.  If  to  the  time 
required  for  the  making  of  these  rocks  we  add  the  long  lapses 
of  time  represented  by  the  various  unconformities,  it  becomes 
evident  that  the  Proterozoic  era  was  one  of  the  longest.  By 
comparing  the  thicknesses  of  younger  systems  it  has  been  esti- 
mated that  it  may  have  been  as  long  as  all  the  subsequent 
periods  combined. 

LIFE  IN  THE  PROTEROZOIC  ERA 

Evidence  from  the  sediments.  —  In  the  Archseozoic  era 
living  things  are  believed  to  have  been  present,  but  the  evi- 
dence of  their  existence  is  somewhat  indirect,  for  no  fossils 
have  been  found.  Again,  in  the  Proterozoic  systems  of  rocks, 
we  find  limestones,  this  time  in  thick  layers,  which  may  have 
been  made  partly  of  the  shells  of  minute  animals,  just  as 
more  recent  limestones  have  been. 

It  is  well  known  that  coal  beds  have  been  derived  from 
compressed  masses  of  the  vegetation  which  accumulates  in 
swamps.  Coaly  layers  and  beds  of  graphite  among  the  Pro- 
terozoic rocks  probably  had  the  same  origin.  Still  other  facts 
make  it  almost  certain  that  both  plants  and  animals  were 
abundant  throughout  the  era. 

Fossils  very  rare.  —  Among  the  younger  strata  of  the  Pro- 
terozoic group  a  few  poorly  preserved  fossils  have  been  dis- 
covered. They  are  the  remains  of  animals,  and  among  them 
are  forms  which  seem  to  belong  to  the  brachiopods  (p.  298) 
and  the  crustaceans  (p.  300). 

The  crustacean  group  is  one  of  the  most  advanced  of  all 
the  invertebrates,  and  it  is  therefore  somewhat  surprising 
that  it  should  have  appeared  so  early.  Few  though  they  are, 
these  fossils  justify  us  in  believing  that  the  living  world  had 
been  in  existence  for  untold  ages  before  the  strata  which 
contain  them  were  deposited,  and  that  the  slow  changes  of 
evolution  had  already  produced  some  types  not  altogether 
unlike  those  of  modern  times. 


330 


HISTORICAL  GEOLOGY 


QUESTIONS 

1.  Pebbles  of  Archaean  schist  are  of  ten  found  in  the  basal  layers 
of  the  Proterozoic  rocks.     Under  what  conditions  are  schists  pro- 
duced?    And  what  does  this  tell  about  the  depth  to  which  the 
erosion  of  the  land  had  penetrated  at  this  time  ? 

2.  What  events  are  recorded  by  the  unconformities  in  Figures 
326,327,  and  328? 


FIG.  326.  —  An  irregular 
contact  between  hori- 
zontal  beds. 


FIG.  327.  —  Horizontal 
sandstone  resting  up- 
on  folded  beds. 


u-  328.  —  Horizontal 
sandstone  resting  up- 
on  granite,  schist,  and 
slate. 


3.  With  which  type  of  volcanic  eruption  are  cinders  and  ashes 
usually  associated,  —  the  fissure  or  the  crater  type  ? 

4.  Why  should  the  gravel  beds  and  cindery  layers  of  the  Ke- 
weenawan  contain  richer  copper  ores  than  the  dense  lava  flows  ? 

5.  Why  should  the  older  Proterozoic  formations  be  more  folded 
and  metamorphosed  on  the  average  than  the  younger? 

6.  Beds  of  conglomerate  thought  to  be  of   glacial  origin  have 
recently  been  found  in  theHuronian  rocks  of  Canada.     With  which 
theory  of  the  origin  of  the  earth  is  this  more  harmonious  ? 


CHAPTER  XIII 
THE    CAMBRIAN    PERIOD 

The  Cambrian  rocks.  —  Most  of  the  rocks  which  consti- 
tute the  Cambrian  system  in  the  United  States  were  origi- 
nally sands,  clays,  and  oozes,  deposited  in  nearly  horizontal 
layers  upon  the  bottom  of  the  seas  of  the  Cambrian  time. 
That  portion  of  the  deposits  from  which  the  sea  has  since 
been  withdrawn  and  which  has  been  exposed  to  view  by  the 
removal  of  such  younger  strata  as  were  deposited  on  them, 
was  laid  down  chiefly  in  the  shallow  waters  near  shores.  For 
this  reason  the  clastic  sediments  predominate  in  the  Cambrian 
system  as  we  know  it.  Embedded  in  these  sediments  we  find 
the  shells  of  some  of  the  animals  which  lived  in  the  same 
seas.  The  fossils  in  the  lower  layers  differ  somewhat  from 
those  found  in  the  upper  beds  of  the  system,  and  by  the 
gradual  changes  in  the  fossils  from  level  to  level,  several 
stages,  or  horizons,  have  been  recognized  within  the  Cambrian 
system.  A  threefold  division  of  the  system  is  usually  made, 
giving  us  Lower,  Middle,  and  Upper  Cambrian  series,  corre- 
sponding to  similar  epochs  of  time. 

Basal  unconformity.  —  The  lowest  layers  of  the  Cambrian 
sediments  generally  rest  upon  an  uneven  eroded  surface  of  the 
older  rocks.  In  some  places  the  underlying  strata  are  ot 
Proterozoic  age ;  in  others  of  Archaean  age.  Some  of  the  older 
rocks  were  folded  or  even  metamorphosed  before  the  Cam- 
brian strata  were  laid  down.  As  evidence  of  this,  it  is  common 
to  find,  in  the  lowest  Cambrian  beds,  pebbles  which  are  water- 
worn  fragments  of  the  older  rocks.  The  unconformity  thus 
indicated  has  been  observed  in  many  parts  of  the  continent, 
and,  as  very  few  exceptions  have  been  discovered,  it  is  evi- 
dent that  before  the  Cambrian  period  began,  most  of  North 
America  had  been  for  a  time  dry  land  and  subjected  to  ero- 

33l" 


332  HISTORICAL  GEOLOGY 

sion.  Where  the  eroded  surface  of  the  older  rocks  has  not 
been  deformed  by  later  movements  of  the  crust,  it  is  nearly 
level ;  and  from  this  fact  it  is  thought  that  the  denudation  of 
the  continent,  before  the  land  was  submerged  by  the  Cam- 
brian sea,  must  have  continued  for  a  very  long  time,  —  suffi- 
ciently long  to  allow  the  streams  to  reduce  large  areas  to  the 
condition  of  peneplains  (p.  147).  On  account  of  the  great 
duration  of  this  interval  of  erosion,  and  because  of  the  very 
general  presence  of  the  unconformity  in  all  continents  where 
the  Cambrian  has  been  studied,  the  interruption  is  regarded 
as  one  of  the  greatest  in  the  geologic  record. 

Gradual  submergence  of  the  continent.  —  Further  light  is 
cast  upon  the  geography  of  the  times  by  the  discovery  that, 
in  North  America,  the  layers  which  contain  the  oldest  Cam- 
brian fossils  exist  only  near  the  eastern  and  western  borders 
of  the  continent.  Farther  inland  it  is  the  Middle  Cambrian 
that  rests  on  the  eroded  pre-Cambrian  surface;  and  in  the 
interior,  from  New  York  to  Michigan,  the  strata  above  the 
unconformity  contain  the  Upper  Cambrian  fossils.  From 
this  we  infer  that  the  sea  encroached  so  slowly  upon  the 
gently  inclined  land  surface  that  nearly  the  whole  of  the  long 
Cambrian  period  was  required  to  accomplish  the  submergence. 
Not  all  of  the  continent  seems  to  have  disappeared  beneath 
the  sea  even  at  this  time.  A  large  area  of  ancient  rocks  in 
eastern  Canada,  another  occupying  what  is  now  the  Atlantic 
seaboard,  and  also  some  parts  of  the  West  seem  to  have 
remained  as  land  masses.  These  continued  to  be  eroded  and 
hence  to  supply  sediments  to  the  seas  of  the  time.  The 
name  "  Appalachia  "  is  used  to  designate  the  large  island 
which  then  lay  just  east  of  the  present  Appalachian  Moun- 
tains, from  New  England  to  the  Gulf  states.  Its  influence 
on  the  rocks  formed  in  later  periods  will  be  mentioned  in 
succeeding  Chapters. 

Where  seas  have  encroached  upon  the  land,  it  is  often  im- 
possible to  decide  whether  the  ocean  surface  actually  rose  or 
whether  the  lands  sank.  In  the  Cambrian,  it  is  significant 


THE  CAMBRIAN  PERIOD  333 

that  the  sea  advanced  gradually  and  almost  simultaneously 
over  central  Europe  and  eastern  Asia,  as  well  as  North  Amer- 
ica. Since  continents  can  hardly  be  supposed  to  subside 
evenly  over  so  large  a  portion  of  the  globe,  the  facts  in  this 
case  suggest  a  general  rise  of  the  ocean  waters.  The  very 
sediments  which  were  being  carried  into  the  sea  all  through 
the  Cambrian  period  would  inevitably  displace  a  considerable 
amount  of  water,  and  raise  the  sea  level  correspondingly. 

Cambrian  strata  differ  according  to  locality.  —  The  Cam- 
brian rocks  are  by  no  means  alike  in  all  localities,  for  the  con- 
ditions of  sedimentation  varied  from  place  to  place.  Where 
the  sea  advanced  over  a  low  shelving  surface  its  waves  and 
currents  reworked  the  soils  and  alluvial  deposits  already  pre- 
pared by  weathering  and  wash,  and  sifted  from  them  an 
abundance  of  sand  which  was  spread  widely  along  the  shores. 
This  may  be  the  explanation  of  the  very  widespread  Middle 
and  Upper  Cambrian  sandstone  which  represents  the  system 
wherever  it  is  exposed  in  the  interior  of  the  United  States. 
In  the  West,  and  in  the  Appalachian  Mountains,  the  deposits 
of  Upper  Cambrian  age  are  principally  limestones  and  shales, 
indicating  that  in  those  districts  conditions  for  clastic  sedi- 
mentation along  shore  had  passed.  During  much  of  the 
period  the  water  may  have  been  too  deep  to  receive  the  coarser 
sediments,  but  it  is  more  probable  that  the  lands  were  low 
and  remote,  and  were  for  that  reason  unable  to  furnish  much 
debris.  Owing  to  the  differences  in  the  conditions  of  sedi- 
mentation and  in  the  time  it  continued,  the  total  thickness 
of  the  Cambrian  strata  is  in  some  places  great  and  in  others 
small.  Over  the  flat  interior  region  the  sea  was  apparently 
shallow  and  came  in  late  in  the  period,  so  that  the  sandy  for- 
mation (Potsdam  sandstone)  then  produced  is  rarely  more 
than  one  thousand  feet  thick.  In  some  places,  on  the  other 
hand,  as  in  the  Appalachian  Mountains  and  in  Nevada, 
deposition  of  varying  sediments  seems  to  have  continued 
nearly  or  quite  throughout  the  period,  and  to  have  resulted 
in  a  succession  of  strata  several  thousand  feet  in  depth. 

B.  &  B.  GEOL.  —  19 


334 


HISTORICAL  GEOLOGY 


Shifting  of  volcanic  activity.  —  In  striking  contrast  to  the 
Keweenawan  system,  the  Cambrian  rocks  of  North  America 
contain  scarcely  a  trace  of  volcanic  materials.  As  later 
periods  are  studied  it  will  be  seen  that  volcanic  activity  is 
prevalent  in  one  region  for  a  time  and  then  dies  out,  only  to 
break  forth  again  in  some  other  district.  So  in  the  Cambrian 
period,  Wales  and  Scotland,  to-day  entirely  without  volcanic 
activity,  were  the  scenes  of  many  eruptions. 

Later  changes  in  the  Cambrian  rocks.  —  The  sediments  of 
which  we  have  sketched  the  origin  have  since  been  changed 

in  various  ways. 
Almost  all  have 
been  converted  into 
firm  rocks :  the  ooze 
into  limestones,  the 
muds  into  shales, 
and  the  sands  into 
sandstones  or  even 
quartzites.  Along 
both  the  Atlantic 
and  Pacific  coasts 
they  have  been  in 
part  metamor- 
phosed into  slates,  schists,  and  gneisses  by  exceptional  com- 
pression and  at  the  same  time  their  fossils  were  obliterated. 
Wherever  the  sea  is,  there  sediments  are  being  deposited ; 
and  to  these  must  be  added  the  debris  laid  down  in  lakes  and 
other  low  places.  The  rocks  of  any  period  therefore  originally 
formed  a  layer  somewhat  more  extensive  than  the  seas  of  their 
time.  Most  of  that  blanket  of  rock  which  we  call  the  Cambrian 
system  is  still  beneath  the  sea  or,  if  raised  above  it,  remains 
concealed  by  the  formations  afterwards  laid  upon  it.  Around 
the  borders  of  the  old  Cambrian  lands  the  system  now  out- 
crops in  an  irregular  band  adjacent  to  the  older  rocks,  and,  in 
certain  mountain  regions  both  east  and  west,  the  Cambrian 
has  been  exposed  by  the  deep  erosion  of  raised  or  folded  tracts. 


FIG.  329.  —  Block  diagram  of  a  dome  fold  like  that 
of  the  Black  Hills  of  South  Dakota,  showing  the 
relation  of  the  Cambrian  (solid  black)  and  later 
sedimentary  rocks  to  the  highly  folded  rocks  of 
pre-Cambrian  age. 


THE   CAMBRIAN  PERIOD  335 

Cambrian  life  highly  developed.  —  The  existence  of  life 
in  the  earlier  pre-Cambrian  periods  of  the  earth's  history  is 
known  only  from  the  indirect  evidence  of  organic  sediments 
and  the  like,  or  from  the  testimony  of  a  few  imperfect  fossil 
shells.  In  the  Cambrian  rocks,  for  the  first  time,  we  find 
such  shells  abundant  and  varied  in  form.  It  would  not  be 
unnatural  to  expect  that  these  early  animals  and  plants  would 
prove  to  be  very  primitive  in  their  structure  and  low  in  the 
scale  of  evolution,  —  but  such  is  not  the  case.  Of  the  eight 
or  nine  primary  divisions  of  the  animal  kingdom,  all  but  the 
highest,  the  vertebrates,  have  Cambrian  representatives.  It 
is  probably  not  too  much  to  say  that  more  than  one  half  of 
the  development  of  the  animal  kingdom  was  accomplished 
before  the  Cambrian.  We  thus  get  a  hint  of  the  long  ages 
which  preceded  the  time  of  which  the  geologic  record  gives  us 
an  intelligible  story.  In  spite  of  the  great  development  of 
life  before  the  Cambrian,  enormous  progress  was  made  in  the 
later  periods,  and,  as  compared  with  the  animals  which  suc- 
ceeded them,  the  Cambrian  types  show  many  primitive 
characteristics. 

Plants  existed.  —  Concerning  the  plants  of  the  Cambrian 
time,  little  is  known ;  but  since  plants  provide  the  ultimate 
food  supply  of  most  animals,  it  is  evident  that  they  must 
have  been  then  in  existence.  We  may  perhaps  attribute 
the  lack  of  fossils  to  the  fact  that  the  Cambrian  rocks  thus 
far  studied  are  of  marine  origin,  and  most  marine  plants  are 
too  soft  and  succulent  to  be  readily  preserved  as  fossils.  Only 
when  in  younger  strata  we  come  to  the  deposits  made  in 
marshes  and  rivers  by  the  plants  which  possess  woody  tissues, 
do  we  find  vegetable  remains  well  preserved. 

The  more  prominent  animals  of  the  Cambrian.  —  Two 
groups  of  animals,  the  brachiopods  and  the  trilobites,  have 
left  fossil  remains  in  such  abundance  that  they  are  regarded 
as  the  most  important  of  all  that  numerous  assemblage  of 
species  which  is  called  the  Cambrian  fauna.  The  early 
brachiopods  had  pairs  of  small  oval  or  rounded  shells  which 


336 


HISTORICAL  GEOLOGY 


FIG.  330.  —  a  and  b.  A  Cam- 
brian brachiopod.  Interior 
and  exterior  views  of  the  shell. 


are  commonly  ornamented  only  by  concentric  lines  of  growth 

(Fig.   330).     Internally   they   exhibit  the   simplest  type   of 

brachiopod  structure,  the  spiral 
feeding  arms  not  supported  by 
hard  skeletons,  and  hence  not 
preserved,  and  the  two  shells  held 
together  by  muscles  only,  rather 
than  by  a  solid  hinge. 

The    trilobites     had    attained 
somewhat  greater  variety  of  form 

even  before  the  Cambrian  period  began, 

and  were    seemingly    more    advanced    in 

their    cycle     of    evolution.      Some     very 

simple  types    (Figs.   331   and   332)    were 

present,   species    which  were   eyeless   and 

had  only  two  body  segments  between  the 

broad  head  and  tail.     Others  were  of  large    FIG.  331 

size  (even  exceeding  two  feet  in  length,  in 

exceptional  instances)  and  were  ornamented 

with  spines  and  raised  lines  (Figs.  333  and 

334).     Most  of  them  possessed  prominent 

compound  eyes  not  unlike  those  of  insects, 

and  they  were  provided  with  a  generous 

number  of  jointed  legs  of  a  type  adapted 

to  swimming.     These  crustaceans,  by  vir- 
tue of  their  advantage   in   size  and  their 

greater  intelligence  and  activity,  doubtless 

held  the   dominant   place   in   the  animal 

world  of  their  day.     Many  other  groups, 

such  as  the  corals,  mollusks  (Figs.  335  and 

336),   worms,   and    graptolites,   have   left 

representatives  among  the  fossils  of  the 

Cambrian  strata,  but  they  scarcely  attained 

prominence  until  later  periods.     As  yet  we  have  no  knowledge 

of  the  existence  in  the  Cambrian  of  air-breathing  animals,  such 

as  insects,  nor  of  even  the  simplest  vertebrates. 


One  of 
the  earliest  and 
simplest  trilobitea 
(Agnostus),  char- 
acteristic of  the 
Cambrian  rocks. 


FIG.  332. — A  larger 
Cambrian  trilobite 
(Conocoryphe). 

Compare  this 
with  Silurian  va- 
rieties. In  which 
are  the  eyes  visible  ? 


THE   CAMBRIAN  PERIOD 


337 


A  large 
trilobite(Olenellus) 
characteristic  of 
the  lower  Cam- 
brian rocks. 


Climate   of  the    Cambrian   period.  —  In 

the  days  when  the   Laplacian  or  gaseous 

theory  of  the  earth's  origin  was  generally 

accepted  as  true,  it  was  thought  that,  in  a 

period  so  remote  as  the  Cambrian,  the  at- 
mosphere must  have  been  distinctly  warmer, 

more  moist,  and  more  heavily  charged  with 

carbon  dioxide  than  now.     There  was  no 

direct  evidence,  however,  that  such  condi- 
tions  really  existed,  and  in   more  recent 

years   some    facts   have    been   discovered 

which  effectually  show  that  they  did  not.    FIG.  333. 

Glacial   deposits   of    early  Cambrian    age 

exist    in    Norway,   China,    and    probably 

elsewhere.      In  China  the  glaciers  were  not 

far  from  sea  level  in  about  the  latitude  of 

New  Orleans.    From  this  it  is  reasonable  to 

infer  that  the  general  climate  of  the  earth 
in  the  Cambrian  pe- 
riod was  not  radically 
different  from  that 
which  prevails  at 
present. 

Close  of  the  period. 
—  The  Cambrian  sys- 
tem is  somewhat  ar- 
bitrarily set  off  from  the  Ordovician 

FIG.  335.  —  Supposed    because   of  a  difference  in    the    fossils 

HtheTlnSha1SbHIyof    which  the  rocks  contain.     It  is  probable 
Cambrian  shale.  that,  when  the  history  of  the  two  periods 

is  better  known,  a  more  rational  means 
of  separation  will  be  found. 

Estimates  of  the  length  of  the  Cam- 
FIG.  336.  — A  cap-    brian  period.  —  There  is  no  satisfactory 

fromPetheSacSambrian    means    of  determining   the   number  of 
system  (Stenotheca).    years  in   any  of   the   geologic   periods. 


FIG.  334. — A  large 
trilobite  (Dikelo- 
cephalus)  charac- 
teristic of  the  late 
Cambrian  rocks. 


338 


HISTORICAL  GEOLOGY 


Nevertheless  calculations,  based  chiefly  on  the  thickness  of 
sediments  deposited,  give  a  rough  approximation  to  the  truth, 
sufficient  to  show  that  geologic  history  is  exceedingly  long. 
It  has  been  estimated  that  from  2,000,000  to  3,000,000  years 
would  be  necessary  for  the  deposition  of  the  sand,  mud,  and 
ooze  which  formed  the  thick  Cambrian  strata.  Similar  es- 
timates made  for  later  periods  indicate  that  the  majority  of 
them  were  of  some  such  duration.  Their  combined  length 
must  then  have  been  many  millions  of  years,  a  lapse  of  time 
almost  too  vast  for  comprehension. 

QUESTIONS 

1.  How  can  the  extent  of  the  sea  at  a  particular  time  in  geo- 
logic history  be  ascertained  ? 

2.  Why  should  limestone  be  deposited  close  to  the  shore  of  a 
low,  densely  forested  land,  but  not  near  a  rugged  or  less  verdant 
coast  ? 

3.  Why  should  the  Cambrian  system  be  thicker  on  the  average 
where  it  is  made  up  of  sandstone  and  conglomerate  than  where  it 
consists  largely  of  limestone  ? 

4.  Over  which  kind  of  a  surface  would  a  rising  sea  spread  most 
rapidly,  a  peneplain  or  a  mountainous  plateau  ?     Why  ? 

5.  At  several  points  in  the  interior  of  the  United  States  the 
basal  layers   of   the    Cambrian   sandstone   contain   great   angular 
bowlders  of  quartzite,  granite,  and  other  rocks.      What  do  these 
indicate  about  the  Cambrian  shore  line  at  those  particular  places  ? 


FIG.  337. 


FIG.  338. 


6.  Compare  the  diagrams,  Figures  337  and  338.  In  which  do  you 
find  evidence  of  the  existence  of  an  island  in  the  Cambrian  period  ? 
Can  you  suggest  how  the  other  has  come  to  resemble  it  in  general 
structure  ? 


CHAPTER  XIV 
THE    ORDOVICIAN   PERIOD 

Expansion  of  the  sea  in  North  America.  —  By  the  end  of 

the  Cambrian  period  the  sea  had  overspread  the  greater  part 
of  North  America.  Neglecting  certain  retreats  and  read- 
vances  of  this  sea,  the  salient  fact  is  that  the  general  submer- 
gence seems  to  have  been  greatest  during  the  Ordovician 
period  (Fig.  339),  gradually  giving  place  to  the  reverse  tend- 
ency toward  the  close.  On  the  east  side  of  the  continental 
sea  lay  the  island  of  Appalachia,  an  extensive  land  stretching 
from  New  England  to  the  Gulf  states  entirely  east  of  the 
present  Appalachian  ranges.  Westward  from  this  island  an 
open  sea  spread  over  the  interior  of  the  continent,  probably 
joining  the  Pacific.  Some  interrupting  islands,  whose  out- 
lines are  imperfectly  known,  are  thought  to  have  existed  in 
the  western  part  of  the  country.  On  the  north  lay  other 
lands  now  represented  by  the  ancient  rocks  of  eastern  Canada 
and  adjacent  parts  of  the  United  States.  That  much  of  this 
sea  was  shallow  is  indicated  by  the  remains  of  corals  of  the 
reef-making  type  and  other  animals  which  to-day  are  unable 
to  live  in  deep  water.  Such  a  shallow  body  of  salt  water 
lapping  up  over  the  continent  is  termed  an  epicontinental  sea. 
Many  single  species  of  Ordovician  fossils  are  found  alike  in 
Europe  and  in  the  United  States,  a  fact  which  seems  to  mean 
that  it  was  possible  for  the  animals  of  the  shallow  waters  to  mi- 
grate freely  from  one  continent  to  the  other.  As  some  of  these 
animals  find  it  almost  as  difficult  to  cross  the  deep  parts  of 
the  ocean  as  to  pass  a  barrier  of  dry  land,  we  may  suppose 
that  the  shallow  sea  which  spread  over  parts  of  Canada  was 
directly  connected  with  the  similar  sea  of  northern  Europe. 

339 


340 


HISTORICAL  GEOLOGY 


FIG.  339.  —  Approximate  distribution  of  land  and  sea  in  North  America  in 
the  middle  of  the  Ordovician  period.     (Modified  after  Willis.) 

Sedimentation  under  varying  conditions.  —  In  different 
parts  of  this  interior  sea  the  conditions  were  not  alike,  and 
hence  the  sediments  are  not  the  same  in  different  localities. 
Over  the  great  central  and  western  interior  region  limy  ooze, 
composed  partly  of  the  shells  of  animals,  was  the  most  impor- 
tant sediment,  and  there  we  now  find  thick  beds  of  limestone. 


THE   ORDOVICIAN  PERIOD  341 

This  implies  clear  water,  for,  although  shell-bearing  ani- 
mals are  often  abundant  in  turbid  waters,  their  remains  are 
there  mixed  with  so  much  mud  or  sand,  that  shale  or  sand- 
stone is  the  resulting  rock.  Thus  along  the  western  flank 
of  Appalachia  there  is  less  limestone  in  the  Ordovician  system 
because  the  land  supplied  greater  quantities  of  sand  and  clay. 

Where  lands  are  high  they  are  more  rapidly  eroded,  and 
when  the  mountains  are  near  the  sea  a  correspondingly  rapid 
accumulation  of  coarse  sediments  is  likely  to  take  place  off 
shore.  When,  however,  broad  low  plains  clad  with  vegeta- 
tion border  the  seas,  it  may  happen  that  little  material  is  worn 
from  the  surface  thus  protected,  and  likewise  little  sediment 
may  be  washed  into  the  sea  in  that  vicinity.  Such  considera- 
tions as  these  serve  to  explain  the  fact  that  the  period  is 
represented  by  over  4000  feet  of  strata  in  eastern  Ten- 
nessee, but  by  only  a  few  hundreds  of  feet  in  Missouri. 
Similarly,  there  may  be  differences  in  the  rate  at  which  cal- 
careous sediments  accumulate,  for  in  warm,  shallow  waters 
shell-bearing  animals  are  likely  to  be  far  more  numerous  than 
in  cold  waters  and  far  from  shore. 

Subsequent  changes  in  the  sediments.  —  The  Ordovician 
sediments  were  laid  down  in  nearly  horizontal  beds,  and  were 
almost  entirely  buried  by  sediments  deposited  at  a  later  time. 
Since  then  they  have  been  consolidated  into  hard  sandstone, 
limestone,  and  shale.  In  some  places  they  have  been  folded 
or  bulged  up  in  such  a  way  that  they  have  been  uncovered 
by  the  erosion  of  the  land.  Thus  the  outcrops  of  Ordovician 
rocks  are  now  found  adjacent  to  those  of  Cambrian  age.  In 
the  Appalachian  Mountains  these  outcrops  lie  in  parallel 
bands,  while  on  the  other  hand  they  form  rings  about  certain 
upraised  masses  of  older  rocks  in  the  northern  and  western 
states,  as  in  the  Adirondacks,  in  Missouri,  and  in  the  Rocky 
Mountains.  On  the  Pacific  coast,  as  well  as  in  New  England, 
the  Ordovician  rocks  have  been  severely  metamorphosed,  so 
that  it  is  now  a  matter  of  extreme  difficulty  to  distinguish  them 
at  all 


342 


HISTORICAL  GEOLOGY 


Lead  and  zinc  deposits.  —  In  parts  of  the  Mississippi 
valley  ores  of  lead  and  zinc  are  now  found  abundantly  in  the 
Ordovician  limestone.  Apparently  minute  particles  of  lead 
and  zinc  minerals  were  deposited  sparsely  through  the  sedi- 
ments while  they  were  accumulating,  and,  at  a  later  time,  these 
scattered  particles  were  dissolved  out  by  the  waters  which 
saturate  the  rocks,  and  were  redeposited  along  joints  and 

bedding  planes  in  the  lime- 
stone (Fig.  340).  Thus 
concentrated  in  veins,  the 
minerals  may  be  profitably 
mined. 

Wide  distribution  of  the 
sea  life. — The  broad,  shal- 
l°w  seas  °f  the  Ordovician 
period  afforded  a  congenial 
home  for  many  species  of 
marine  organisms,  and,  al- 
though it  is  certain  that  the 
majority  of  the  forms  which 
existed  then  have  left  no 
traces  in  the  rocks,  yet 
enough  have  been  preserved 
to  show  us  the  variety  and 
advancement  of  the  ani- 
mals of  the  time.  The  wide 
expansion  of  the  seas,  and 
the  free  communication  which  seems  to  have  prevailed  be- 
tween them,  permitted  the  individual  species  to  migrate 
readily  from  one  part  of  the  globe  to  another.  This  was  par- 
ticularly true  of  animals  which  floated  in  the  water,  such  as 
graptolites  and  young  corals  (p.  296).  Hence  some  of  the 
Ordovician  fossils  of  the  United  States  are  much  like  those 
of  Europe  and  even  Asia  and  Australia.  Such  a  widespread 
assemblage  of  animals  is  called  a  cosmopolitan  fauna.  In 
any  one  place  the  animals  of  Ordovician  time  were  in  part 


FIG.  340.  —  Vertical  section  of  a  zinc- 
and-lead  ore  deposit  in  southwestern 
Wisconsin.  (After  Chamberlin.) 


THE   ORDOVICIAN  PERIOD 


343 


istic  Ordovician  brach- 
iopod  (Orthis). 


descended  from  those  which  lived  there  in  the  Cambrian 
period,  and  in  part  from  others  which 
had  come  in  from  elsewhere. 

Progress  of  the  brachiopods  and  trilo- 
bites. —  Among  the  members  of  this 
fauna  the  brachiopods  and  trilobites  still 
held  a  prominent  position.  The  little  Fig.  341.— A  character- 
oval  varieties  of  the  former  were  at  this 
time  associated  with  larger  types,  many 

of  which  were  ornamented  with  radiat- 
ing ridges  (Fig.  341).  The  species  that 
had  hinged  shells  were  more  numerous 
and  even  the  spire-bearing  group  was 
represented  (Fig.  342).  During  the 
Ordovician  period  the  trilobites  had 

FIG.   342. — A   common      .  .  . 

Ordovician  brachiopod  risen  rapidly  to  their  culmination,  and 
with  hooked  beak  were  even  more  numerous  than  in  the 

(Rhynchotrema).  _.        .     .  _     „ 

Cambrian.  As  we  shall  see,  it  was  not 
until  the  next  period,  however,  that  they  ex- 
hibited to  the  fullest  their  propensity  for  adopt- 
ing queer  forms  and  orna- 
ments. Some  of  the  Or- 
dovician trilobites  went 
to  the  extreme  of  sim- 
plicity (Fig.  343)  in  their 
adornment;  a  few  are 
quite  smooth,  and  are  all 
but  devoid  of  even  the 
pair  of  furrows  (Fig.  344) 
which  impart  to  most 
members  of  the  group 
their  trilobate  aspect. 


FIG.  343.  — A 
remarkably 
smooth  trilo- 
bite  (Bumas- 
tus)  from  the 
Ordovician 
rocks. 


telus)  of  the  Ordovi-  addition  to  the  brachiopods  and  trilo- 

cian  period.   Compare  j^es,  other  groups  rose  to  prominence 

TUG   GyGS  Wltll    TJIlOSG  OI 

Cambrian  types.  in  the  Ordovician.     Some  were  repre- 


344 


HISTORICAL  GEOLOGY 


sented  in  a  subordinate  role  in  the  Cambrian  fauna,  while 
others  seem  to  have  made  their  appearance  after  the  close  of 
the  period.  Of  these  none  is  more  important 
than  the  graptolites  (Figs.  346  and  347), 
those  colonies  of  little  polyps  strung  on 
stems.  Being  freely  float- 
ing animals  they  were 
easily  transported  by  ocean 
currents,  and  hence  single 
species  had  an  almost 
world-wide  range.  Their 
relatives,  the  corals,  here 
became  important  for  the 

first    time.       It     is    to     be    FIG.  346.  —  A  colony 

noted  that  in  the  early 
stages  of  their  evolution 
the  corals  were  represented 
chiefly  by  the  solitary 
hornlike  forms  (Fig.  348), 
FIG.  345.— Head  whereas  the  habit  of  living  in  compact  colo- 

pLacLenoidS^f  nies  (FiS-  349)  became  ™re  prevalent  in 
the  Ordovician  later  periods,  until  to-day  the  compound 
period.  corals  far  outnumber  the  solitary  varieties. 

The  mollusks  (p.  298),  of  which  only  the  pteropods  and 
cap-shaped  gastropods  had  been  noteworthy  in  the  Cambrian, 


of  graptolites.  Each 
little  tooth  on  the 
blades  held  an  indi- 
vidual polyp.  The 
central  portion  may 
have  served  partly 
as  a  float. 


FIG.  347.  —  A  branching  colony  of  graptolites  impressed  upon  a  piece  of  shale. 


THE  ORDOVICIAN  PERIOD 


345 


subsequently  expanded  into  considerable  diversity. 
tropods  developed  many 
variations  of  the  spiral 
(Fig.  350)  and  flat-coiled 
shells  (Fig.  351),  lacking 
in  fact  only  the  orna- 
mental spines  and  tu- 
bercles of  our  modern 
species.  The  two- 
-  shelled  mollusks,  or  pe- 

FIG.    348.  —  A   small 

horn  coral  (Strepte-  lecypods,  seem  to  have 

lasma)  from  the  Or-   macje     slower    progress  I 
do  vician  limestones.  ' 

yet  there  are  many  of 
them  in  certain  Ordovician  rocks  (Fig. 
352).  Like  the  brachiopods,  they  first  ap- 
peared with  simple  unornamented  shells, 
gathering  complexity  of 
structure  and  decora- 
tion as  they  advanced. 
The  highest,  and  in  some 
respects  the  most  re- 
markable, group  of  mol- 
lusks,  the  cephalopods, 

rrmlrAQ    if«s    fir«+    smnpnr 

makes  its  nrst  appear- 
ance  in  numbers  in  the 
Ordovician  strata.  The 
earliest  types  had 
FIG.  350.-AnOrdo-  straight  tapering  shells 
(Fig.  353),  open  at  the 

d  divided 

partitions 


The  gas- 


FIG.  349.  — Broken 
fragment  of  one  of 
the  earliest  com- 
pound corals,  show- 
ing several  coales- 
cent  tubes  each 
built  by  an  individ- 
ual coral  animal. 


stout 

flat-coiled  gastro- 
pod (Bellerophon) 
common  in  the  Or- 
dovician period. 


vician  gastropod 

(Hormotoma)   with    j 
tall,  spiral  shell. 

by    sagging 


into  a  series  of  chambers. 

vance  is  shown  in  curved  (Fig.  354)  or  even       >CyP°d 


A  seeming  ad-  FIG.  352.  — A  small, 

plain     pelec: 
(Ctenodonta). 


tightly  coiled  shells  (Fig.  355),  which  ap- 
peared at  this  time.     The  remarkable  folding  of  the  dividing 
partitions  did  not,  however,  set  in  until  the  Devonian. 


346 


HISTORICAL  GEOLOGY 


There  is  evidence  that  the  great  vertebrate  branch  had 
become  distinct  as  early  as  the  Ordovician  period,  for  scales, 
which  appear  to  be  those  of  fishes,  have  been 
found  in  rocks  of  that  age  in  the  Rocky 
Mountains.  Still  another  long  period  must 
be  passed,  however,  before  fishes  come  into 
prominence. 

Land  plants  and  animals. — Considering  the 
fact  that  the  continents  were  so  largely  sub- 
merged and  that  the  known  Ordovician  strata 
in  which  our  only  record  of 
the  life  is  preserved  are  of 
FIG.  353.  — One  marme  origin,  it  is  not  sur- 

of    the  earliest         .   .        ,,,{,.,       j        .        , 

and  simplest  prising  that  the  land  animals 
cephaiopods  anc[  plants  of  this  period  are 

(Orthoceras).  ,       ,  ,  ,, 

scarcely  better  known  than 
are  those  of  the  Cambrian.  An  insect's 
wing  from  the  rocks  of  Sweden  proves  that 
the  land-inhabiting  arthropods  had  already 
come  into  being,  and  it  adds  confirmation  to 

Our      previous    FIG.  354. — Broken 


suspicion  that 

land     vegeta-     alopod     (Cyrto- 

tirm  PYiVpH  in        ceras)>    ComPare 
the  sutures  with 

those      early       those  in  Figures 

times;  for  the      391  and 418. 
winged     insects     are    almost 
wholly  dependent  upon  plants 
for  their  sustenance. 

Crustal   disturbances  at  the 
close  of  the  period. — The  long, 
quiet    reign   of   the    epiconti- 
nental  seas,  which  had  begun 
in  the  Cambrian  and  continued 
FIG.    355.  — A   coiled    Ordovician   through  the  Ordovician,   was 
cephalopod  related  to  forms  still 

living.  partially  interrupted  by  events 


THE   ORDOVICIAN  PERIOD  347 

which  are  used  to  mark  the  close  of  the  latter  period.  It  is 
believed  that,  during  ages  of  tranquillity  of  the  earth's  sur- 
face, the  forces  which  at  times  produce  warping  and  moun- 
tain folding  accumulate  power  until  finally  the  resisting 
strength  of  the  rocks  is  overcome,  and  the  outer  layers  are 
wrinkled  and  broken.  This  wrinkling  is  usually  confined  to 
a  small  belt  or  district,  but  within  that  area  the  folding 
and  crushing  may  be  intense.  In  the  present  instance  the 
first  premonition  of  a  change  is  afforded  by  the  fact  that 
the  clear  seas  of  the  Middle  Ordovician  in  eastern  United 
States  later  became  turbid  with  mud,  so  that  the  last 
strata  of  the  system  are  shales  overlying  the  limestones. 
Evidently  changes  in  the  activities  of  rivers  or  currents,  or 
both,  were  in  progress,  although  it  is  not  easy  to  prove  just 
what  the  changes  were.  In  eastern  New  York  the  early 
Silurian  strata  are  found  lying  unconformably  upon  highly 
folded  rocks  which  are  known  to  be  of  Ordovician  age. 
From  this  it  is  known  that  the  recently  deposited  Ordovician 
and  older  strata,  in  that  region  and  somewhat  farther  south- 
ward, were  intensely  deformed ;  and  also  that  the  same  region 
became  land  and  was  subject  to  long-continued  erosion. 
The  wide  extent  of  the  unconformity  shows  that  much  of 
the  eastern  interior  of  the  United  States  emerged  at  the  same 
time. 

During  the  compression  of  the  rocks  in  the  East,  shales 
became  schists,  and  fossil-bearing  limestone  was  altered  to 
marble  in  which  nearly  all  trace  of  fossils  has  disappeared. 
The  local  nature  of  this  disturbance  becomes  evident  when  it 
is  found  that  in  the  adjacent  regions  of  New  York  and  New 
Jersey  the  Ordovician  rocks  were  only  slightly  disturbed  at 
this  time,  while  in  some  portions  of  the  Mississippi  Basin  they 
did  not  even  emerge  from  the  sea.  The  obvious  result  of  the 
folding  must  have  been  a  belt  of  mountains,  perhaps  of  notable 
height.  Although  these  have  since  been  totally  cut  away  by 
the  erosive  agencies,  their  site  is  occupied  by  the  newer 
Taconic  Mountains  of  to-day,  and  so  this  disturbance  which 


348  HISTORICAL  GEOLOGY 

closed  the  Ordovician  period  is  frequently  spoken  of  as  the 
"  Taconic  revolution." 

Similar  events  in  Europe.  —  In  Europe  the  deposition  of 
sediments  in  Ordovician  time  was  in  many  ways  like  that  in 
the  United  States,  and  at  the  close  it  suffered  a  similar  inter- 
ruption. The  rocks  of  Wales  and  Scotland  were  highly  folded 
into  a  series  of  mountains  which  were  gradually  worn  down 
during  the  Silurian  period.  The  fact  that  the  crust  was 
simultaneously  wrinkled  on  both  borders  of  the  Atlantic  Ocean 
suggests  that  a  slight  subsidence  of  the  great  oceanic  area  may 
have  been  directly  responsible  for  the  disturbance.  Yet  it 
cannot  be  said  that  this  is  proved. 

QUESTIONS 

1.  Sun  cracks  have  been  found  on  the  bedding  planes  of  the 
Lower  Ordovician  limestone  in  the  Mississippi  Valley.     From  this, 
what  do  you  infer  as  to  the  depth  of  the  water  in  which  this  lime- 
stone was  deposited  ?     How  does  this  compare  with  limestones  in 
general  ? 

2.  Judging  from  what  you  know  of  the  Archaean  and  Algonkian 
systems,  what  was  the  character  of  the  rocks  from  which  the  Ordo- 
vician sediments  were  derived  ?     Why  does  the  Ordovician  system 
consist  of  limestone,  shale,  and  sandstone,  rather  than  pieces  of  these 
older  rocks  cemented  together  ? 

3.  What  is  the  chief  process  of  change  at  work  on  the  surface 
of  a  land  which  is  too  low  to  be  eroded  by  streams  ? 

4.  Can  you  suggest  why  nothing  is  known  about  the  sediments 
which  were  deposited  in  the  Ordovician  period  off  the  eastern  shore 
of  Appalachia  ? 

5.  What  phase  of  metamorphism  would  be  most  likely  to  obliter- 
ate all  traces  of  fossils  in  the  Ordovician  rocks  of  the  Taconic  Moun- 
tains ? 

6.  Can  you  see  any  reason  for  thinking  that  vertebrates  were 
in  existence  long  before  the  fishes  whose  plates  have  been  found  in 
the  Ordovician  rocks  ? 


CHAPTER  XV 
THE    SILURIAN   PERIOD 

Transition  from  Ordovician  to  Silurian.  —  In  eastern  United 
States  and  western  Europe  the  Ordovician  period  seems  to  be 
distinctly  set  off  from  the  Silurian  by  the  so-called  Taconic 
revolution.  Elsewhere,  however,  the  transition  from  the 
one  to  the  other  was  quiet  and  not  marked  by  notable  disturb- 
ances. Some  portions  of  this  continent  emerged  from  the  sea 
and  became  low  plains,  from  the  surfaces  of  which  little  debris 
could  be  eroded.  In  Oklahoma,  on  the  other  hand,  and  in 
the  western  states  generally,  the  surface  appears  to  have 
remained  submerged  beneath  the  sea.  These  things  are 
clearly  shown  by  the  succession  of  the  sedimentary  rocks. 
Thus,  as  mentioned  on  a  preceding  page,  the  Silurian  strata 
lie  in  marked  unconformity  upon  the  folded  Ordovician  rocks 
in  the  New  England  region.  In  Tennessee,  Minnesota,  and 
some  other  states,  the  two  systems  are  parallel  in  bedding, 
but  are  separated  by  an  irregular  weathered  surface  which 
is  in  reality  an  unconformity.  In  Utah  and  Montana  the 
Silurian  system  is  only  a  part  of  a  thick  succession  of  lime- 
stones which  contain  Ordovician  fossils  below  and  Devonian 
fossils  above. 

Clastic  sediments  along  the  eastern  land.  —  The  oldest 
sediments  referred  to  the  Silurian  period  are  unlike  in  different 
parts  of  the  country.  Along  the  western  flank  of  the  newly 
made  eastern  highlands  quantities  of  gravel  and  sand  brought 
down  by  swift  rivers  were  spread  out  in  thick  banks  which 
thinned  toward  the  west.  The  gravel,  now  consolidated  into 
hard  conglomerate,  is  known  as  the  Oneida  formation.  Where 
it  has  since  been  tilted  up  on  edge  it  forms  mountain  ridges, 
because  the  softer  rocks  on  each  side  of  it  have  been  more 
B.  &  B.  GEOL.  —  20  349 


350  HISTORICAL  GEOLOGY 

rapidly  removed  by  erosion.  The  sand  and  finer  sediments 
sifted  from  the  gravel  were  carried  farther  westward,  forming 
the  Medina  sandstone.  As  the  high  lands  were  worn  down, 
the  rivers  became  less  active,  and  less  gravel  was  strewn  along 

the  front  of  the 
mountains.     As 

FIG.  356.  — Diagram  showing  the  relation  of  the  the  zone  of  gravel 
Silurian  limestone  in  the  Mississippi  Valley  to  the  accumulation  be- 
conglomerate,  sandstone,  and  shale  in  New  York. 

came  narrower, 

the  zone  of  sand  deposition  encroached  upon  it,  and  it  thus 
happened  that  the  Medina  sands  extended  continually  farther 
and  farther  eastward  until  they  came  to  lie  partly  upon  the 
Oneida  beds  (Fig.  356). 

The  Clinton  iron  formation.  —  In  the  more  remote  parts  of 
the  interior  this  rejuvenation  of  the  New  England  region 
seems  to  have  exerted  no  influence.  In  Illinois,  for  example, 
the  first  Silurian  beds  were  of  shale  and  limestone,  and  the 
deposition  continued  without  change  in  the  character  of  the 
sediments  until  the  latter  part  of  the  period.  Between  the 
sandy  coastal  plain  and  this  clear,  open  sea  there  was  an 
irregular  belt  over  which  sediments  rich  in  compounds  of  iron 
were  deposited  on  a  large  scale.  This  phase  of  the  Silurian 
rocks  has  been  named  the  Clinton  formation.  The  iron  ore 
is  usually  of  the  red  variety  or  hematite;  in  some  places, 
where  massive  beds  several  feet  in  thickness  are  found,  pro- 
ductive iron  mines  are  located.  The  microscope  shows  that 
some  of  this  ore  has  the  structure  of  limestone,  —  that  is,  the 
rock  is  composed  of  bits  of  shells,  corals,  etc.,  but  the  material 
is  largely  iron  oxide  instead  of  lime  carbonate.  Students  of 
the  subject  are  not  yet  agreed  as  to  the  exact  conditions  under 
which  these  unusual  deposits  were  made,  but  there  seems  good 
reason  to  believe  that  the  sediments  were  laid  down  in  shallow 
water  not  far  from  land. 

In  the  process  of  smelting  iron  ore  it  is  mixed  with  limestone 
and  coke,  and  when  the  mixture  is  heated  in  the  furnace,  the  iron 
is  released  from  the  ore  and  flows  out  into  the  molds.  At  Bir- 


THE  SILURIAN  PERIOD 


351 


mingham,  Alabama,  Clinton  iron  ore,  coal,  and  limestone  are  found 
together.  This  fortunate  combination  has  made  that  region  one 
of  the  great  centers  of  the  iron  and  steel  industry,  and  a  place  of 
much  importance  in  the  industrial  upbuilding  of  the  southern  states. 

The  interior  sea  again  enlarged. —  As  the  period  progressed, 
the  sea  seems  to  have  encroached  slowly  upon  the  land,  much 
as  it  did  during  the  Cambrian.  One  broad  arm  extended 
northward  across  Canada  and  perhaps  into  the  polar  regions. 
As  the  lands  were  worn 
lower  and  the  shores  ad- 
vanced eastward  in  the 
United  States,  the  zones 
of  deposition  migrated 
accordingly,  so  that  not 
only  did  the  Medina 
sandstone  come  to  over- 
lap the  Oneida  conglom- 
erate, but  the  limestone 
of  the  West,  with  its 
peculiar  iron-bearing 
shoreward  phase,  over- 
spread the  Medina  as 
far  as  central  New  York. 
From  the  fact  that  its 

massive  layers  form  the    FlG-  357-  —  Diagram  of  a  limestone  cliff  in 
r£C  i  •  i  Montana,  showing  levels  (x)  at  which  fos- 

Cllff      Over     which      the        sils  of  different  ages  were  found. 

Niagara  River  plunges 
in  its  famous  cataract, 
the  limestone  is  known  as  the  Niagara  formation.  It  is,  of 
course,  much  thicker  in  the  Mississippi  Valley,  where  it  seems 
to  have  accumulated  through  most  of  the  period,  than  in  the 
New  York  region,  where  it  began  to  be  deposited  considerably 
later.  The  Silurian  furnishes  an  illustration  of  the  well-known 
fact  that  a  single  rock  formation  in  one  part  of  the  country 
may  be  equivalent  in  time  to  several  distinct  and  unlike 
formations  in  another  place. 


How  may  the  absence  of  Silurian  fossils 
be  explained  ? 


352  HISTORICAL  GEOLOGY 

In  western  North  America  the  Silurian  system,  where 
found,  consists  of  limestone.  It  is  in  fact  merely  a  part  of  a 
thick  limestone  series  which  contains  faunas  characteristic 
of  the  Ordovician  and  Devonian  periods  as  well  as  of  the  Si- 
lurian. The  implication  is  plain  that  for  long  periods  of  time 
the  open  sea  held  uninterrupted  sway.  In  the  region  in- 
cluding Colorado  and  part  of  Wyoming, 
however,  Silurian  rocks  are  unknown  and 
it  is  possible  that  here  was  a  land  mass 
in  Silurian  time. 

Animals  of  the  Niagaran  sea.  —  Our 
knowledge  of  the  living  things  of  the 
Silurian  is  largely  confined  to  the  rich 
and  varied  society  of  animals  which  in- 
habited the  clear  though  shallow  seas  of 
the  time.  The  Oneida  conglomerate  has 
yielded  no  fossils,  and  the  Medina  very 
few,  —  perhaps  because  the  turbulent 

FIG.   358.  —  One  of   the          ,  i     i     i      i  i_«  v    j-   >    M 

commonest    triiobites  and  sand-choked  streams  which  distrib- 
(Caiymene)  of  the  Ni-  uted    them    were    not     attractive     to 
aquatic  animals.     The   Niagara   fauna, 
then,  may  be  considered  by  itself. 

Of  the  groups  mentioned  in  discussing  the  Ordovician  period 
all  but  two  made  notable  progress  in  the  Silurian,  the  excep- 
tions being  the  graptolites  and  the  triiobites.  The  decline  of 
the  graptolites  from  their  position  of  importance  in  the  pre- 
ceding period  was  rapid.  They  are  not  numerous  even  in  the 
Niagara  rocks,  and  the  Devonian  period  witnessed  their 
complete  extinction.  Among  the  triiobites,  however,  the 
descent  from  supremacy  was  more  gradual.  In  the  Silurian 
they  were  still  abundant,  and  never  were  they  more  diversi- 
fied in  form  than  at  this  time.  Like  the  decadent  nations 
revealed  to  us  in  human  history,  they  indulged  in  extrava- 
gant and  futile  eccentricities,  ill  befitting  their  approaching 
overthrow.  Odd  and  highly  ornate  forms  appeared  in  pro- 
fusion (Figs.  359,  360,  and  361),  and  in  most  instances  the 


THE   SILURIAN  PERIOD 


353 


spines,  tubercles,  and  horns  which  they  produced  seem  to 
have  had  little  or  no  real  value  in  their  life  activities.    We 


FIG.  359.— An  unu- 
sually spiny  trilo- 
btte  (Acidaspis)  from 
the  Silurian  of  Bo- 
hemia. 


FIG.  360. — Atrilobite 
(Lichas)  of  the  Si- 
lurian period.  Com- 
pare with  Cambrian 
trilobites. 


FIG.  361.— A  highly 
specialized  Silurian 
trilobite  of  peculiar 
form  (Deiphon). 


shall  see  in  studying  the  later  periods  that  similar  eccentric- 
ities mark  the  fall  of  other  groups,  such  as  the  ammonites 
and  the  reptiles. 

Among  the  rising  groups 
only  a  few  require  special 
mention.  The  corals 
show  an  increase  in  the 
number  of  composite 
types,  such  as  the  "  honey- 
comb coral"  (Fig.  362) 
and  the  "  chain  coral " 
(Fig.  363),  as  against  the 
horn  corals.  Although 
the  echinoderms  had  been 
represented  even  as  early 
as  the  Cambrian  and  had  attained  some  importance  in  the 
Ordovician,  they  did  not  reach  commanding  prominence  until 
the  Silurian.  The  clear,  shallow  seas  in  which  the  Niagara 
ooze  was  produced  furnished  congenial  life  conditions  not  only 
for  corals  but  for  communities  of  the  crinoids  (Fig.  364),  — 


FIG.  362. — A  piece  of  honeycomb  coral 
(Favosites). 


354 


HISTORICAL  GEOLOGY 


graceful  animals  attached  to  the  sea  floor  by  flexible  stalks 
and  provided  with  feathery  arms  or  tentacles  around  the 
mouth  (p.  297).  The  mollusks  (Figs.  365 
and  367),  and  their  companions  the  brachi- 
opods  (Figs.  366,  368,  and  369),  developed 
steadily  along  the  lines  already  defined  in 
earlier  times,  and  be- 
came constantly  more 
numerous.  Clearly 
preserved  fishes  appear 

FIG.  363.  —  The  chain    f          ,       ,,      £        *\. 
coral    (Halysites),    here  for  tne  first  time> 

common  in  Silurian  but  as   yet   they  are 
rare,  and  the  considera- 
tion of  them  may  best  be  deferred  until 
the   Devonian  is   discussed.     They  were 
extreme^  primitive 
types,  unlike  any  that 
are  now  living. 

Life  on  the  lands.  — 
The  plants,  which  we 

FIG.   365.-A  stout  ma^  wel1    believe 
Silurian    gastropod  clothed    the    Silurian 

(Strophostylus).  j^    are   almogt   un_ 

known,  —  doubtless  for 
the  same  reason  that 
has  been  suggested  to 
explain  the  similar  ab- 
sence of  information 
about  the  flora1  of 
earlier  periods  :  the 

FIG.  366,-  Alarge  and    knOWn  r°cks  afe  chief^ 

strongly  beaked  of    marine    origin. 

brachiopod  (Conchi-    Equally    scant     ig    the 

J 


FIG.  364. — A  nearly 
perfect  crinoid,  as 
found  in  the  Niagara 
limestone  of  Indiana. 
The  roots  served 
merely  for  attach- 
ment. 


dmm)  of  the  Silurian 

period.  record  of   air-breathing  arthropods,  but 

1  The  flora  of  a  country  or  of  a  period  is  the  entire  assemblage  of  trees, 
shrubs,  herbs,  and  other  plants  living  in  that  place  or  time. 


THE  SILURIAN  PERIOD 


355 


FIG.  367.  —  A  coiled  Silurian  ceph- 
alopod  (Phragmoceras). 


the  fossils  already  discovered  show  that,  even  before  the  Silu- 
rian period,  this  group  had  become  divided  into  its  constituent 

classes,  such  as  insects,  scor- 
pions, and  others. 

Relations  with  Europe.  —  A 

fauna  very  similar  to  that  just 

described  lived  in  a  sea  which 

occupied  the  site  of  England 

and  the  Baltic  region  during 

the  same  time.     It  is  thought 

that  the  route  of  intermigra- 

tion   between  the  two  conti- 
nents lay  along  a  shallow-water 

tract     which     extended     up 

through   Canada  and  Alaska 

and  perhaps  even   the   polar 

regions.      So    easy    was    the 

communication  along  this  path  that  peculiar  Swedish  corals 
and  trilobites  found  their  way  over  to 
Iowa,  and  crinoids  characteristic  of  the 
United  States  became  residents  also  of 
England. 

Silurian  deserts.  —  The  quiet  continu- 
ance of  these  broad  epicontinental  seas 
(Rhynchotreta)  char-   was  interrupted   in   both  continents   by 
changes  of  far-reaching  importance.     The 
deposition  of  limestone  in  eastern  United 

States    gradually   ceased,   and,    in   some 

areas,  if  not  in  all,  this  was  occasioned 

by  the  emergence  of  the  sea  bottom  into 

a  low-lying  land.     In  the  West  at  this 

time  much  of  the  region  from  Montana 

south  westward  remained  under  water.        FlG  369  _A  giiurian 
In  the  East  the  Niagara  limestone  is      brachiopod    (Ortho- 

frequently  found  lying  unconformably  be-     *f J^  %* 

neath  the  later  deposits.     In  the  districts      long  hinge  line. 


Flpointed  " 


ri<anIrotcks?f 


356  HISTORICAL  GEOLOGY 

adjacent  to  lakes  Erie  and  Ontario,  sediments  continued  to 
be  deposited.  While  part  of  the  beds  were  laid  down  under 
water,  this  was  evidently  not  the  water  of  the  open  sea.  The 
rocks  (Salina  beds)  consist  of  shales  and  sandstones  of  reddish 
and  gray  colors  interbedded  with  seams  of  gypsum  and  rock 
salt.  The  salty  beds  are  covered  by  a  peculiar  limestone,  parts 
of  which  are  valuable  for  the  manufacture  of  hydraulic  cement, 
and  in  this  limestone  are  found,  not  the  Niagara  fossils,  but 
peculiar  arthropods  (Fig.  370)  and  fishes 
of  types  which  are  almost  unknown  in 
strictly  marine  formations. 

The  Silurian  salt  beds  of  New  York  have 
long  furnished  a  large  part  of  the  salt  used 
in  this  country.  Wells  have  been  bored 
through  the  overlying  strata  into  the  salt 
beds,  and  the  salty  water  is  pumped  to  the 
surface.  There  the  water  is  evaporated  and 
the  salt  remains. 

At  the  present  time  beds  of  salt  and 
gypsum  are  produced  in  excessively  salt 
lakes,  such  as  Great  Salt  Lake  and  the 
FIG   370  —A  large  ar-  Dead  Sea.     These  saline  lakes  are  con- 
thropod  related  to  those  fined  to  desert  regions  where  evaporation 

WatlL™  ton!    is  raPid'      U  is  ^&^t  also  that  the 

sediments  deposited  in  some  desert  basins 
are  of  a  red  or  brownish  color.  From  these  considerations  it 
appears  that,  in  the  late  Silurian,  northeastern  United  States 
had  a  distinctly  arid  climate.  Most  deserts  are  now  situated 
in  the  interiors  of  continents,  either  where  they  are  sheltered 
from  moist  winds  by  barrier  mountain  ranges,  or  where  drying 
winds,  like  the  trade  winds,  blow  constantly.  The  emergence 
of  the  continent  which  seems  to  have  occurred  in  the  late 
Silurian  largely  increased  the  area  of  land,  and,  if  highlands  of 
sufficient  elevation  were  so  situated  as  to  exclude  the  moist 
winds  from  the  Gulf  of  Mexico  and  the  Atlantic,  which  now 
bring  rain  to  the  Ontario  region,  the  conditions  for  local 


THE  SILURIAN  PERIOD 


357 


deserts  would  have  been  present.  The  upper  limestone,  or 
Water-lime  formation,  is  thought  to  have  been  deposited 
partly  in  fresh  or  brackish  lakes,  which  were  perhaps  made 
possible  by  an  increase  in  the  rainfall  of  that  region. 

Closing  incursion  of  the  sea.  —  In  the  vicinity  of  lakes 
Erie  and  Ontario,  where  the  Salina  beds  are  best  known,  the 
Water-lime  formation  grades  upward  into  limestone  with  coral 
reefs  and  marine  shells.  A  second  incursion  of  the  epiconti- 
nental  sea  is  thus  recorded.  Between  these  Monroe  strata, 
as  they  are  called,  and  the  overlying  Devonian  rocks  there  is 
no  sharp  dividing  line,  but  merely  a  gradual  change  in  the 
kinds  of  fossils. 

QUESTIONS 

1.  Why  are  the  divisions  of  the  Silurian  system  as  recognized 
in  New  York  not  suitable  for  Illinois  ? 

2.  The  pebbles  of  the  Oneida  formation  consist  largely  of  pure 
quartz.     Can  you  suggest  how  pure  quartz  gravel  could  be  derived 
from  a  complex  mass  of  igneous  and  metamorphic  rocks  such  as 
those  which  were  exposed  in  the  ancient  continent  of  Appalachia? 

3.  By  what  process  may  loose  gravel  be  transformed  into  a 
hard  rock  capable  of  forming  mountain  ridges  ? 

4.  Why  should  fossils  be  rare  in  the  Oneida  formation,  even  if 
shell-bearing  animals  were  abundant  at  the  time  and  place  it  was 
deposited  ? 

5.  At   Cobalt   Lake,   in   northern   Ontario,   the  Niagara  lime- 
stone lies   directly  upon   the   surface  of   Huronian   and   Archaean 
igneous  rocks.     What  different  hypotheses  may  account  for  this 
relation  ? 

6.  Small  patches  of  Niagara  limestone  are  found  northeast  of 
the  edge  of  the  continuous 

formation  in   Canada  and 

the  United  States.     What 

is  the  significance  of  these 

outliers  (Fig.  371),  as  they  FlG"  371.— Diagram  of  outliers. 

are  called,  with  reference  to  the  former  distribution  of  the  Silurian 

system  ? 


CHAPTER  XVI 
THE   DEVONIAN    PERIOD 

Relations  to  the  Silurian.  —  In  North  America  the  Silurian 
and  Devonian  systems  are  not  sharply  separated  from  each 
other,  either  by  a  striking  unconformity  or  by  noteworthy 
changes  in  the  character  of  the  sediments.  For  this  reason 
there  has  been  some  dispute  as  to  where  the  division  should  be 
made.  The  fact  serves  to  illustrate  the  general  principle 
that  geologic  time  itself  is  unbroken  and  that  the  divisions 
which  we  recognize  must  necessarily  be  somewhat  arbitrary 
and  local  in  their  application. 

At  the  close  of  the  Silurian  period  the  great  central  part  of 
North  America  seems  to  have  been  land.  In  many  parts  of 
the  country,  —  for  example,  northern  Illinois,  Alabama,  and 
Colorado,  —  no  sediments  of  earlier  Devonian  age  exist,  and 
it  is  thought  that  much  of  this  area  was  land  at  that  time.  In 
some  other  places,  as  in  Iowa,  an  unconformity  has  been 
found  at  the  base  of  the  Devonian  system.  The  detection  of 
this  interruption  is  usually  difficult,  inasmuch  as  the  beds 
below  are  parallel  with  those  above;  upon  careful  examina- 
tion, however,  the  irregularity  of  the  contact,  the  slightly 
weathered  surface  of  the  uppermost  Silurian  beds,  and  the 
abrupt  change  in  the  fossils  serve  to  prove  the  existence  of 
the  break.  Such  an  unconformity  clearly  indicates  two 
things,  namely,  that  the  older  rocks  were  not  deformed,  as 
were  those  of  New  England  at  the  close  of  the  Ordovician 
period,  and  that  when  the  sea  withdrew  it  left  a  land  surface 
of  very  slight  relief.  Had  the  land  been  high  above  the  sea, 
the  rivers  would  have  cut  deeply  into  it  and  would  either  have 
developed  a  very  hilly  surface,  or,  if  the  erosion  cycle  had 
gone  on  to  old  age,  the  Silurian  strata  would  have  been 

358 


THE  DEVONIAN  PERIOD  359 

largely  carried  off  and  the  Devonian  sea  would  have  en- 
croached upon  a  plain  underlain  by  still  older  rocks. 

North  America  at  the  beginning  of  the  Devonian  period.  — 
Although  this  low-lying  land  seems  to  have  stretched  from 
Michigan  and  Virginia  westward  over  much  of  the  present 
Mississippi  Basin,  the  sea  had  by  no  means  entirely  retreated 
from  the  continent.  In  what  is  now  the  lower  Great  Lakes 
region  and  again  in  Utah,  Nevada,  and  Montana,  the  deposi- 
tion of  limestone  and  shale  went  on  from  the  Silurian  far  into 
the  Devonian.  Considering  the  isolation  of  these  localities,  it  is 
not  surprising  that  the  fossils  in  the  one  place  bear  little  rela- 
tion to  those  of  the  other.  No  more  do  the  animals  which 
inhabit  the  seas  off  California  and  New  England  to-day. 

DEVONIAN  IN  THE  WEST 

As  the  Devonian  period  progressed,  the  events  in  one  region 
were  not  necessarily  the  same  as  those  in  another.  In  Utah, 
for  example,  lime  ooze  and  mud  were  deposited  uninterrupt- 
edly throughout  the  period,  with  the  result  that  limestone 
about  1000  feet  thick  now  represents  the  Devonian  in  that 
region.  So  free  from  disturbing  influences  was  this  part 
of  western  United  States  that  the  animals  of  the  western 
sea  underwent  only  very  slow  changes.  The  fossils  in  the 
youngest  beds  of  the  system  do  not  seem  to  differ  widely 
from  those  in  the  oldest.  The  conditions  elsewhere  were  in 
contrast  to  this. 

DEVONIAN  IN  THE  EAST 

Helderberg  limestone.  —  In  eastern  United  States  the 
period  was  marked  by  the  gradual  reexpansion  of  the  epicon- 
tinental  sea,  attended  by  important  changes  in  the  relations 
of  the  land  and  water  bodies.  At  first  all  the  eastern  lands 
seem  to  have  been  low,  for  if  land  masses  had  been  eroded 
rapidly,  the  derived  sediments  would  surely  have  formed 
clastic  rocks  in  the  adjacent  seas.  As  it  was,  only  limestone 


360 


HISTORICAL  GEOLOGY 


was  laid  down  and  that  chiefly  in  a  restricted  sea  extending 
from  the  St.  Lawrence  region  to  Virginia.  This  is  called  the 
Helderberg  limestone  because  it  is  well  exhibited  in  the  Hel- 
derberg  Mountains  of  eastern  New  York.  At  the  same  time, 
apparently,  a  bay  extended  up  from  the  south  into  Tennessee 
and  Indian  Territory. 

Oriskany  sandstone.  —  As  the  clear  sea  with  its  limey 
bottom  spread  slowly  westward  into  the  Mississippi  Valley 
and  perhaps  south  to  Alabama,  some  radical  change  in  the 
middle  Atlantic  states  allowed  coarse, 
sandy  sediments  to  be  spread  out  over 
the  Helderberg  formation.  The  result- 
ing Oriskany  sandstone  is  several  hun- 
dreds of  feet  thick,  and  the  sand  of  which 
it  is  made  represents  the  decomposition 
of  a  vast  amount  of  solid  rock.  (How 
might  this  be  accounted  for  (1)  by  cli- 
matic change,  (2)  by  diastrophism  ?) 

Onondaga  limestone.  —  Gradually  the 
deposition  of  sands  became  restricted, 
and  the  sea  which  occupied  the  Appa- 
lachian depression  was  again  clear.  In 

FIG.    372.  —  A    bit    of     .,  ,    ,.  „     ,    ,,       ~ 

organ-pipe  coral  (Sy-    ^  a  second  limestone,  called  the  Onon- 
ringopora)  from  the    daga,  was  deposited  over  the  Oriskany 
sandstone.     The  warmth   and  shallow- 
ness  of  the  Onondaga  sea  are  shown  by  the  abundance  of 
corals   (Fig.  372)   and  other  animals  which   frequent   coral 
reefs.     By  this  time,  also,  the  northwestern  part  of  the  con- 
tinent, from  Alaska  to  Alberta,  was  covered  by  the  waters 
of   the   northern    ocean.     They    were  apparently  not  cold 
waters  in  those  days,  for  reasons  not    yet  well  understood. 
Hamilton     shales.  —  Muddy     sediments     succeeded     the 
Onondaga  deposits  in  the  East,  and  even  in  Illinois  the  lime- 
stone is  less  pure.     The  Hamilton  shales  are  usually  dark  and 
bituminous,  implying  an  abundance  of  minute  plants  as  well 
as  the  animals  whose  shells  abound  in  the  same  beds.     The 


THE  DEVONIAN   PERIOD  361 

slow  decay  of  these  organisms  is  believed  to  have  produced 
most  of  the  petroleum  and  gas  which  are  now  obtained  from 
the  Devonian  rocks  in  Ontario,  Ohio,  and  Pennsylvania. 
From  the  fact  that  the  Hamilton  formation  thickens  as  it  is 
followed  eastward  it  seems  probable  that  the  mud  was  largely 
derived  from  lands  along  the  present  Atlantic  slope. 

The  basins  coalesce.  —  Toward  the  close  of  the  Devonian 
the  epicontinental  sea  attained  still  greater  extent.  By  the 
spreading  of  the  northwest  Canadian  sea  southward  and  east- 
ward, the  western  and  eastern  basins  of  the  United  States 
seem  to  have  been  joined  (Fig.  373).  In  the  West  and  North- 
west muds  and  oozes  continued  to  be  deposited,  and  from  this 
we  may  infer  that  in  those  remote  times  the  western  part  of 
America  had  none  of  its  present  rugged  mountain  ranges,  but 
that  it  was  a  flat  or  undulating  lowland.  As  the  upper 
Devonian  strata  are  traced  eastward  to  Ohio  and  beyond, 
they  become  increasingly  thicker  and  more  sandy.  We  have 
already  learned  that  a  change  from  ooze  to  mud  and  thence 
to  sand  is  to  be  expected  as  one  approaches  the  shore  line. 
The  Chemung  formation,  as  these  sandy  shales  are  called, 
grades  finally  into  thick  sandstones  which  contain  few  fossils 
except  leaves  of  plants  and  bones  of  fishes.  It  is  from  these 
non-marine  strata  that  the  Catskill  Mountains  have  been 
carved.  The  imperfect  assortment  of  the  sediments  suggests 
that  they  were  strewn  by  rivers  rather  than  by  waves,  and 
that  the  Catskill  beds  represent  the  alluvial  apron  built  out 
into  the  shallow  sea  on  the  west  by  streams  which  descended 
from  the  highlands  of  the  Appalachian  continent. 

MIGRATIONS  AND  CHANGES  OF  THE  SEA  LIFE 

As  the  continuance  of  the  clear  sea  over  a  comparatively 
isolated  province  such  as  the  Nevada  region  allowed  the 
animals  which  lived  there  to  develop  quietly  along  their  own 
lines  of  advance,  so,  on  the  other  hand,  the  shifting  relations 
of  land  and  sea  and  changing  character  of  the  sediments  in 


362 


HISTORICAL  GEOLOGY 


eastern  United  States  afforded  conditions  for  the  rapid  and 
conflicting  evolution  of  species.     At  the  outset  of  the  period 


FIG.  373.  —  Supposed  geography  of  North  America  in  late  Devonian  time. 
The  dotted  pattern  represents  sediments  on  land.  The  limits  of  the  land 
mass  north  and  south  of  the  United  States  are  wholly  unknown. 


the  animals  of  the  sea  were  confined  to  the  edges  of  the  con- 
tinent and  such  bays  as  lapped  over  its  surface.     Being  iso- 


THE  DEVONIAN  PERIOD  363 

lated,  they  developed  independently,  and  after  the  lapse  of 
sufficient  time  became  notably  different  in  the  several  embay- 
ments.  As  the  sea  later  spread  over  the  land  these  distinct 
faunas  invaded  the  interior  region  from  different  directions, 
—  one  from  the  northeast,  another  from  the  south,  another 
from  the  northwest,  and  so  on.  As  the  widening  seas  mingled, 
the  faunas  were  one  by  one  brought  into  conflict,  much  as  the 
invasion  of  North  America  by  the  French  and  English  brought 
them  into  opposition  in  Canada  and  the  Mississippi  Valley 
in  the  eighteenth  century.  Just  as  we  now  have  a  mixed 
French-English  people  in  Quebec,  so  the  mingling  of  the 
Hamilton  fauna  of  the  East  with  the  McKenzie  fauna  of  the 
Northwest  produced  a  mixed  race  in  which  the  influence  of 
the  Northwest  immigrants  was  strongest,  and  left  its  stamp 
on  the  result.  The  commingling  of  two  marine  faunas  results 
in  something  more  than  a  mere  mixture  of  the  two.  The 
struggle  between  two  faunas  usually  crowds  into  extinction 
certain  weaker  members  of  each  assemblage,  and  it  often 
results  in  the  rapid  rise  of  entirely  new  forms  not  found  in 
either  of  the  original  faunas. 

In  late  Devonian  times  the  result  of  this  succession  of  immi- 
grations and  intermixtures  was  a  fairly  cosmopolitan  fauna 
inhabiting  the  seas  from  Alabama  to  Alaska  and  having  close 
relations  with  the  animals  of  distant  China  and  Russia. 

Changed  conditions  of  life.  —  From  what  has  already 
been  said  of  the  Devonian  faunas  and  their  migrations  it 
will  be  readily  inferred  that  the  fossils  are  abundant  and 
locally  well  preserved.  The  same  groups  which  were  impor- 
tant in  the  Silurian  are  represented  also  in  the  next  period, 
although  with  different  relative  standings.  In  the  Silurian 
the  animals  of  the  clear  seas  were  almost  the  only  forms 
extensively  preserved.  Our  best-known  Devonian  rocks  are, 
however,  chiefly  shales  and  sandstones,  and  so  the  fossils  in 
them  tell  us  of  the  animals  which  frequented  the  mud  banks 
and  the  sandy  shores  rather  than  the  clear,  open  sea.  Con- 
ditions which  are  congenial  for  one  group  of  animals  may  be 


364 


HISTORICAL  GEOLOGY 


FIG.  374.  —  Two  common 
Devonian  pelecypods. 


adverse  or  even  fatal  to  another.  Thus  many  of  the  mollusks 
prefer  somewhat  turbid  water  and  a  muddy  bottom,  while 
the  corals  are  exterminated  by  any 
large  admixture  of  sediment  in  the 
water  in  which  they  live.  With  this 
principle  in  mind  we  shall  be  prepared 
to  find  the  crinoids,  corals,  and  other 
animals  which  were  abundant  in  the 
Niagara  sea  relatively  uncommon  in 
the  Devonian  formations  except  the 
limestones. 

Mollusks  and  brachiopods  numerous. 
—  Their  places  were  taken  by  hosts  of 
two-shelled  mollusks  (Fig.  374)  and 
brachiopods  (Figs.  375  and  376),  with 
other  groups  in  subordinate  positions. 
The  brachiopods  in  particular  were  prob- 
ably near  their  zenith  in  the  Devonian. 
Most  of  the  important  types  had  made 
their  appearance  in  full  force,  and  it  re- 
mained for  later  periods  only  to  carry  out 
the  lines  of  progress  already  defined. 

Decline  of  the  trilobi  es. — The  trilobites 
had  by  this  time  dwindled  to  a  few  forms 
(Fig.  377)  which,  however,  clung  to  their 

Silurian  propensity  for  useless  excres- 
cences and  ornaments.     Although  the 
two  cases  may  not  be  similar,  there  is 
a    resemblance    to   certain   decadent 
families  among  our  own  race  who  cling 
to  the  traditions  and  outward  appear- 
FIG.  376.  — A  large  brachio-  ances  of  former  rank,  long  after  they 
pod  common  in  the  Devo-  have  been  shorn  of  power  and  wealth. 

nian  rocks  (Spirifer).  _      .     t  ,. 

Cephalopods  take  a  new  line  of  ad- 
vance. —  The  chambered  mollusks,  or  cephalopods,  now  enter 
upon  a  new  career  which  eventually  leads  them  to  the  extreme 


Fig.  375. —  One  of 
the  commonest  De- 
vonian brachiopods 
(Atrypa). 


THE  DEVONIAN  PERIOD 


365 


FIG.  377. — Alargetrilo- 
bite  (Dalmanites)  of  the 
Devonian  period. 


of  complexity   and   diversification,   as 

regards  their  internal  structure.    The 

early  Paleozoic  types  had  shells  which 

were  divided  into  chambers  by  a  series 

of  flat  or  saucer-shaped  partitions.     In 

some  of  the    Devonian  species   these 

partitions    became    slightly   folded   at 

their  edges,  and  the  suture  lines  on  the 

outside  of  the  shell  show  corresponding 

lobes  or  angles.     These  simpler  varie- 
ties are  called  goniatites  (Fig.  378) .     It 

will  be  interesting  to   compare  them 

with  the  complex  forms  of  later  times. 
Profusion  of  fishes.  —  Of  all  the  new 

developments     among    the    Devonian 

animals,  none  is  more  important  than 

the   apparently   sudden   rise  of  the   fishes.      From  meager 

beginnings   in  the  previous   periods  they  spread  out  into 

many  different 
types,  and  be- 
came so  abundant 
that  the  Devonian 
is  sometimes 
called  the  Age  of 
Fishes.  Being 
among  the  earliest 
to  make  their  ap- 
pearance, it  is  but 
natural  that  the 
Devonian  repre- 
sentatives of  the 
class  should  have 
been  primitive  in 
their  structure. 
Lowest  in  the 


FIG.  378.  —  A  coiled  cephalopod  (Goniatites)  in  which 
the  sutures  are  slightly  folded. 
B.  &  B.  GEOL.  —  21 


scale  are  the  O.s- 


3(56 


HISTORICAL  GEOLOGY 


tracoderms  (literally  "shell  skin  "),  which  were  not  fishes  at  all, 
in  the  strict  sense  (Fig.  380).  It  is  not  cer- 
tain that  they  possessed  jaws,  but  if  they 
did,  there  is  some  reason  to  think  that  the 
jaws  worked  horizontally  as  in  beetles. 
Strong  resemblances  to  some  of  the  early 
arthropods  are  seen  in  their  bony  head 
shields  and  in  the  closely  spaced  eyes.  In 
fact,  their  claim  to  a  place  among  the 
vertebrates  rests  chiefly  on  the  possession 
of  a  tail  fin  which  seems  to  imply  that 
they  had  a  rudimentary  spinal  column. 
Since  none  are  now  in  existence  it  is  hard 
to  determine  their  real  character. 

The  true  fishes,  which  are  furnished 
with  jaws  of  the  customary  type  and  one 
or  two  pairs  of  fins  along  their  flanks, 
are  represented  in  the  Devonian  fauna  by  many  strange  and 
some  very  large  species.  Some,  on  the  other  hand,  were  not 


FIG.  379.  — A  Devo- 
nian coral,  showing 
$he  cup  with  radiat- 
ing partitions. 


FIG.  380.  —  An  ostracoderm. 

so  unlike  those  of  to-day  that  the  untrained  eye  would  readily 

note  the  difference.     Others  had  the  head  cased  in  heavy 

plates  of  bone,  with  only  the  rear  part  of  the  body  left  in 

a   flexible    condition.      In 

modern    fishes    the    limb 

bones  do  not   extend  out 

into  the  fins,  but  end  in 

blunt  plates  to  which  the 

fin  rays  are  attached  (Fig. 

383).    The  Devonian  fishes, 


FIG.  381.  —  Tail  of  a  primitive  fish  with 
fringe  above  and  below. 


THE   DEVONIAN  PERIOD 


367 


on  the  other  hand,  had  fully 

vertebrated  fins  (Figs.  381  and 

382).     Again,  there  are  some 

very  peculiar  things  about  the 

teeth  of  these  ancient  members 

of  the  finny  tribe.     Unlike  the     FlG.  382.  _  Asymmetrical  tail  of 

sharp,  spikelike  teeth  of  modern 

fishes  (Fig.  385),  many  of  them 

were    rounded    or    corrugated 


the  sturgeon,  in  which  the  body 
axis  follows  the  upper  blade  of 
the  fin. 


plates  (Fig.  384)  adapted  for  grinding 
food  rather  than  for  seizing  live  prey. 

Altogether  the  De- 
vonian fishes  were 
massive  and  clumsy. 
As  in  the  arthropods, 
their  skeletons  were  FlG.  384.-A  single 

chiefly  On   the   Outside        corrugated  tooth  of 

in  the  form  of  bony 

Fro.  383.-Fan-3hIped  armOT>    f°r     the     limb 

tail    fin    characteristic  bones    and   Spinal    COl- 

of  the  higher  types  of  umn   were    often    Uttl 
fishes. 

more    than    cartilage. 
As  time  went  on,  the  advantage  of  speed  pIG  335. —  Pointed 
over    armor   seems    to    have    led    to    the      tooth  of  an  extinct 
strengthening  of  the  internal  skeleton  with 
bone,  and  to  the  development  of  a  more  flexible  body. 


a  Devonian  shark- 
like  fish. 


FIG.  386.  —  A  modern   lung  fish   from  Australia,  not  unlike  certain  Devo- 
nian fishes. 

LIFE  ON  LAND 

For  the  first  time  we  have  among  the  Devonian  fossils  a 
fair  representation  of  the  animals  and  plants  of  the  lands. 


368  HISTORICAL  GEOLOGY 

The  rivers  and  other  land  waters  supported  a  variety  of  fishes 
and  mollusks.  Vegetation  was  luxuriant  in  favorable  places 
and  included  trees  as  well  as  the  lowlier  growths.  The  trees 
of  this  period  were  not,  however,  like  those  of  the  present  day. 
Most  of  them  were  relatives  of  the  ferns.  Insects  and  their 
allies  have  been  found  in  some  numbers.  Considering  the 
small  chance  of  preserving  such  delicate  creatures  in  the  rocks, 
it  would  not  be  reasonable  to  expect  many  fossils. 

QUESTIONS 

1.  Why  is  the  present  distribution  of  the  Devonian  system  less 
than  it  was  originally  ? 

2.  Can  you  suggest  why  the  outcrops  of  the  Devonian  system 
are  usually  narrow  bands  ? 

3.  Oil  is  found  in  Devonian  strata  in  certain  parts  of  eastern 
United  States.     It  is  usually  concentrated  beneath  anticlines.     A 
well  piercing  the  fold  usually  encounters  first  natural  gas,  deeper  oil, 
and  still  farther  down  water.     Can  you  suggest  why  there  should 
be  this  arrangement  ? 

4.  Why  should  the  wells  obtain  more  oil  from  sandstone  than 

___^_^    from  shale  ? 

^J^£J^j£^2lS^'  •  <=>.Q:         5.   An   instance   is   known   of   the 

g^ IT occurrence  of  black  mud  containing 

JL- — I teeth  and  plates  of  Middle  Devonian 

I £  fishes  in  cracks  exposed  in  a  quarry 

_f 3 in  the  Niagara  Limestone    (Fig.  387). 

'  Can  you  suggest  an  explanation? 
FIG.  387. '—  Cracks  in  the  Ni-        Q     Tjiider  wnat  conditions  will  two 

agara    limestone    filled    with  faunag  ^  and   becQme  legg   and 

black  mud  and  fossils.  ,        ...  .      ,1       « 

less  like  each  other .' 

7.  Why  should  the   fossils  of  the   Chemung  formation  be  less 
like  those  of  the  Catskill  beds,  which  are  of  the  same  age,  than 
like  those  of  the  Hamilton  formation,  which  is  distinctly  older  ? 

8.  Can  you  suggest  why  crinoids  and   corals   are  rarely  found 
in  the  Oriskany  formation  ? 

9.  Why  is  it  not  so  easy  to  use  the  small  divisions  of  geologic 
time  in  widely  separated  countries,  as  it  is  to  use  the  larger  divi- 
sions, such  as  eras  ? 

10.  When  a  local  and   a  cosmopolitan  fauna  are  permitted  to 
mingle,  because  of  some  geographic  change,  which  of  the  two  usually 
exerts  the  stronger  influence  on  the  new  fauna  thus  formed,  and  why  ? 


CHAPTER  XVII 
THE    MISSISSIPPIAN   PERIOD 

The  Carboniferous  divided.  —  The  Mississippian,  Pennsyl- 
vanian,  and  Permian  periods  were  formerly  combined  under 
the  name  of  Carboniferous.  Evidence  is  accumulating, 
however,  which  indicates  that  the  three  divisions  are  really 
quite  as  distinct  from  each  other  as  are  such  periods  as  the 
Devonian  and  the  Silurian ;  and  so  it  is  thought  best  to  make 
three  separate  periods  out  of  the  old  Carboniferous.  Each 
is  named  for  a  region  in  which  the  rocks  are  well  exposed  and 
well  known. 

Transition  from  the  Devonian.  —  The  transition  from  the 
Devonian  into  the  Mississippian  period  was  not  marked  by 
abrupt  changes  in  most  parts  of  the  North  American  conti- 
nent. The  chief  event  which  characterizes  the  Mississippian 
is  the  further  expansion,  over  the  greater  part  of  the  United 
States  and  the  Northwest,  of  the  epicontinental  sea  which, 
even  in  the  late  Devonian,  was  fairly  extensive.  This  expan- 
sion of  the  sea  was  followed  later  in  the  period  by  a  corre- 
sponding retreat.  For  the  eastern  interior,  it  was  the  last 
period  of  purely  marine  conditions. 

Clastic  sediments  in  the  East.  —  Over  what  is  now  the 
Mississippi  Basin,  as  far  east  as  Ohio,  and  as  far  west  as 
Nevada  at  least,  was  the  open  sea.  In  that  portion  of 
this  vast  region  which  the  Devonian  ocean  had  also  covered 
the  strata  of  the  two  systems  are  generally  conformable.  In 
much  of  the  West,  however,  the  Mississippian  extends  be- 
yond the  Devonian  and  rests  directly  upon  more  ancient 
rocks,  in  some  places  even  on  the  Archaean.  About  the 
northeastern  border  of  this  sea,  notably  in  Pennsylvania  and 
Ohio,  coarse  sands  and  muds  were  accumulating  rapidly. 

369 


370  HISTORICAL  GEOLOGY 

The  rocks,  as  we  now  find  them,  are  thick  clastic  formations, 
usually  called  the  Pocono  sandstone  below,  and  the  Mauch 
Chunk  shale  above.  Ripple  marks  and  sun  cracks  in  the 
shales  indicate  that  they  were  deposited  in  shallow  water; 
and  a  close  study  of  them  has  recently  made  it  fairly  certain 
that  they  represent  a  great  flat  delta  plain  over  which  rivers 
in  a  semiarid  climate  spread  silts  and  sands  in  times  of 
flood.  Occasional  coal  seams  tell  of  the  existence  of  marshes 
upon  the  surface  of  this  delta  plain. 

Limestone  in  the  central  and  western  states.  —  As  we 
trace  them  farther  west  and  south,  the  land-derived  sediments 
become  finer,  and  limestone  increases  in  prominence.  From 
Indiana  westward  massive  limestones  form  the  bulk  of  the 
Mississippian  system.  The  same  formation  reappears  in  the 
Black  Hills,  parts  of  the  Rocky  Mountains,  and  the  Arizona 
plateaus,  and  is  believed  to  underlie  nearly  all  of  the  Great 
Plains.  This  extensive  limestone  series  implies  a  clear  open 
sea  remote  from  rugged  lands.  That  its  genial  waters 
abounded  in  animals  of  the  sea  is  proved  by  the  crinoids, 
corals,  and  other  fossils  with  which  the  strata  are  locally 
crowded.  This  is  true  especially  in  the  Mississippi  and  Ohio 
Basins,  or,  in  other  words,  near  the  border  line  between  the 
muddy  and  the  limy  bottoms.  The  deep  sea  explorations  of 
the  "  Challenger  Expedition  "  some  years  ago  brought  out 
the  fact  that  animals  are  always  extraordinarily  abundant 
near  the  mud  line  or  the  outer  edge  of  the  muddy  area ;  there 
the  conditions  of  life  seem  to  be  more  favorable  than  else- 
where. 

Sedimentation  outside  of  the  interior  sea.  —  A  body  of 
water  covering  the  southern  peninsula  of  Michigan  was  at 
this  time  more  or  less  isolated  from  the  great  interior  sea. 
The  strata  which  accumulated  there  are  associated  with  salt 
and  gypsum,  suggesting  that  the  local  climate  was  not  moist, 
and  that  the  basin  was  cut  off  from  direct  connection  with 
the  sea. 

Again,  in  Nova  Scotia  sediments  were  laid  down  in  basins 


THE  MISSISSIPPIAN  PERIOD  371 

probably  not  filled  by  the  sea.  Thick  sandstone  and  conglom- 
erate are  there  succeeded  by  shales  with  gypsum. 

Decreasing  seas  at  the  close.  —  The  uppermost  strata 
of  the  Mississippian  in  the  middle  states  are  shaly  and  even 
sandy,  like  the  beds  which  immediately  followed  the  Devonian. 
Above  these  sandy  beds  there  is  usually  a  distinct  unconform- 
ity, which  separates  the  Mississippian  from  the  overlying 
Pennsylvanian  system.  The  lower  shaly  beds  we  have  inter- 
preted as  the  mud  banks  along  the  borders  of  an  expanding 
sea,  in  which  the  deposition  of  mud  was  gradually  being  re- 
placed by  that  of  limy  ooze.  Toward  the  end  of  the  period 
the  sea  was  evidently  being  restricted  in  eastern  United  States. 
As  the  shore  line  migrated  west  and  south,  the  mud  and  sand 
which  are  usually  deposited  near  shores  were  spread  out  over 
the  limestone  that  had  been  deposited  in  the  clearer  sea 
earlier  in  the  period.  Finally  by  withdrawal  of  the  sea  the 
eastern  part  of  the  country  became  land.  The  erosion  of  this 
low  land  was  attended  by  slight  warping  of  the  surface,  and 
even  a  few  faults  and  gentle  folds  were  produced.  The  result 
of  the  disturbance  and  the  erosion  together  is  the  unconform- 
ity between  the  Mississippian  and  Pennsylvanian  systems. 

In  the  far  West,  changes  of  land  and  sea  at  the  close  of  the 
Mississippian  period  were  less  pronounced.  No  distinct  line 
of  separation  between  the  two  systems  has  been  recognized 
in  the  Arizona-Nevada  region ;  but  in  the  Rocky  Mountains 
a  widespread  unconformity  indicates  the  emergence  of  the 
sea  bottom. 

The  Paleozoic  Alps.  —  During  the  four  preceding  periods 
sediments  were  being  deposited  rather  generally  over  western 
Europe,  much  as  in  eastern  United  States.  It  is  noteworthy 
that  Britain  and  Germany  were  also  volcanic  districts  through 
much  of  this  time. 

After  the  close  of  the  Mississippian  period,  these  deposits 
were  locally  folded  up  (Fig.  388)  into  two  great  mountain 
chains,  one  extending  from  Ireland  into  Germany  and  the 
other  from  southern  France  into  Bohemia.  These  folds  now 


372 


HISTORICAL  GEOLOGY 


FIG.  388. 


Trends  of  the  folds  produced  during  the  late  Paleozoic  moun- 
tain-building epoch  in  western  Europe. 


are  so  old  that  they  have  been  worn  down  to  mere  stubs, 
—  such  as  the  low  mountains  and  hills  of  the  Black  Forest 
and  Cornwall.  But  in  their  prime  they  may  well  have  been 
lofty,  snow-capped  ranges,  and  for  that  reason  they  are  styled 
the  Paleozoic  Alps. 

LIFE  OF  THE  MISSISSIPPIAN  SEA 

On  account  of  the  wide  extent  of  the  clear  Mississippian 
sea,  it  is  but  natural  that  of  all  the  life  of  the  period  we  should 
know  the  marine  animals  best.  Very  few  animals  and  plants 
of  the  dry  land  have  escaped  destruction,  nor  are  those  of 
rivers  and  swamps  well  represented  among  the  fossils  of  the 
time. 

Abundance  of  the  crinoids.  —  In  the  limestones  of  earlier 
periods  we  have  noted  the  abundance  of  either  corals  or  cri- 


THE  MISSISSIPPIAN  PERIOD 


373 


FIG.  389. —  One  of 
the  last  represen- 
tatives of  the  trilo- 
bites  (Phillipsia) 
found  in  Mississip- 
pian  rocks. 


iioids  or  both.     They  were  preeminently  animals  of  the  clear 

seas ;    but  the  conditions  of  depth,  temperature,   and  food 

supply  which  are  essential  for  one  are  not 

quite  those  which  are  required  for  the  other. 

In  some  parts  of  the  Mississippian  sea  of 

the  United  States,  corals  seem  not  to  have 

been  favored,  although  they  were  common 

elsewhere.     Where   the    corals    were    few, 

crinoids  were  locally  so  abundant  that  some 

strata  are  composed  mainly  of  their  stems 

and  scattered  plates.     At  no  time  in  their 

history  were  the  crinoids 

more  diversified  or  more 

highly    ornamented. 

Like  the  trilobites  of  the 

Silurian,  some   of   them 

assumed    eccentric    and 

seemingly  useless  changes 

of  form,  with  spines,  ridges,  and  knobs  upon 

the  plates.     Similarly,  the  crinoids  were  at 

this  time  on  the  verge  of  a  rapid  decadence ; 

by  the  close  of  the  Mississippian  period  the 
lGi\39i°i'T      com"  majority    of    them    had    become    extinct, 

plete  blastoid.  One  J         J 

of    the    stemmed   leaving  a    decreas- 

echinoderms,  espe-    mg  line  Qf  descend- 
cially    common  in  . 

the    Mississippian   ants  which  are  but 

limestones.  poorly  represented 

in  our  modern  seas.  The  cause  of 
their  decline  is  yet  a  mystery. 

Development  among  the  fishes.— 
In  the  eastern  part  of  the  interior  sea, 
fishes  were  numerous,  and,  as  we  may 
well  believe,  the  most  formidable 
predatory  animals  of  the  time.  Sig- 
nificant changes  had  taken  place  FlG-  391  -A  Mississippian 

gomatite  with  moderately 

among  them  since  the  Devonian.  The    folded  sutures. 


374 


HISTORICAL  GEOLOGY 


queer  ostracoderms  had  disappeared,  and  the  heavily  protected 
sluggish  types  of  the  true  fishes  were  replaced  by  more  active 
varieties  which  relied  upon  swiftness  rather  than  upon  armor. 

The  place  of  prominence  was 
occupied  by  the  sharks  and 
their  relatives,  but  the  Missis- 
sippian  forms  of  sharks  (Fig. 
392)  were  by  no  means  so  for- 
midable as  their  modern  rep- 
resentatives. In  those  days 
many  of  them  were  provided 
only  with  flat,  corrugated  teeth 
suitable  for  grinding  mollusks  and  other  small  animals.  As 
weapons  of  defense  against  predaceous  fishes,  such  teeth  were 
evidently  not  effective ;  and  perhaps  to  offset  this  lack,  further 
protection  was  added  to  some  varieties  in  the  form  of  sharp 
spines  on  the  outside  of  the  body. 

Advent  of  the  amphibians.  —  The  vertebrates  now  show 
a  distinct  advance  in  the  advent  of  a  class,  some  members  of 


FIG.  392.  —  The  Port  Jackson  shark. 
One  of  the  nearest  living  relatives 
of  some  of  the  Paleozoic  sharks. 
Like  them  its  mouth  is  paved 
with  grinding  teeth. 


FIG.  393.  —  A    modern    salamander    or    tailed    amphibian.     (Jordan    and 

Kellogg.) 

which,  at  least,  were  equipped  with  legs  and  toes,  and  were 
able  to  live  on  land  and  breathe  air.  As  the  vertebrates  are 
now  predominately  land  animals,  this  was  a  notable  step 
toward  the  realization  of  the  future  destiny  of  the  group. 
The  fossil  remains  of  amphibians  are  very  rare  in  the  Mis- 
sissippian  rocks,  and  little  is  known  about  them.  They  were 
long,  salamanderlike  animals  (Figs.  393  and  394),  which 
doubtless  spent  most  of  their  time  in  the  water.  The  rela- 


THE  MISSISS1PP1AN   PERIOD 


375 


tionships  between  these  primitive  amphibians  and  the  fishes 
are  so  close  as  to  leave  small  doubt  that  they  were  actually 
derived  from  one  of  the  groups  of  fishes  in  the  Devonian. 
Indeed,  even  among  the  highest  amphibians  the  young  are 
hardly  more  than  fishes,  breathing  water  through  gills  and 
swimming  by  means  of  fins. 


FIG.  3J4.  —  Larval  form  of  a  salamander,  showing  the  finlike  fringe  on  the 
tail  and  the  branching  external  gills  just  behind  the  head. 

QUESTIONS 

1.  Many  limestones  such  as  those  of  the  Mississippian   system 
contain  nodules  of  flint  and  chert,  —  a  very  dense  form  of  quartz. 
During  the  weathering  of  the  limestone  what  should  become  of 
these  nodules? 

2.  What  different  types  of  animals  could  make  five-toed  foot- 
prints ?    Which  of  these  groups  is  the  lowest  in  the  scale  of  evolution  ? 

3.  An  important  part  of  the  salt  now  used  in  the  United  States 
is  obtained  from  Mississippian  strata.     From  your  knowledge   of 
the  formations  of  this  age,  where  should  you  expect  to  find  the 
center  of  this  salt  industry  ? 

4.  What  structure  should  you  expect  to  find  in  the  Paleozoic 
rocks  of  Belgium  ?     Why  ? 

5.  At  a  locality  in  Illinois  the 
Pennsylvanian  and  earlier  Paleo- 
zoic  systems  have  the  relation 
shown    in    Figure    395.      What 
events  are  indicated  ? 

6.  On  what  grounds  is  it  jus- 


FIG.  395.  —  Section  of  Paleozoic  beds 
in  Illinois. 


tifiable  to  separate  the  Mississippian  as  a  distinct  period? 


CHAPTER  XVIII 
THE   PENNSYLVANIAN   PERIOD 

THE  system  which  contains  the  most  important  deposits  of 
coal  in  both  the  United  States  and  Europe  is  called  in  America 
the  Pennsylvanian.  Because  of  the  great  value  of  the  coal 
beds,  this  division  of  the  old  Carboniferous  has  received  more 
attention  than  the  earlier  and  later  portions. 

Land  interval  at  the  beginning.  —  At  the  close  of  the  Mis- 
sissippian,  a  large  part  of  the  United  States  emerged  from  the 
sea,  and  the  fact  is  recorded  by  an  extensive  unconformity. 
Sediments  continued  to  accumulate  in  certain  low  or  sub- 
merged regions,  for  example  in  Arizona  and  Utah,  and  there 
we  find  transitional  formations;  but  in  the  eastern  interior 
especially,  land  conditions  prevailed.  The  long-continued 
weathering  and  erosion  of  the  land  removed  part  of  the  Mis- 
sissippian  rocks,  and  locally  uncovered  still  older  formations. 
As  the  limestones  crumbled  and  decayed,  a  residual  layer  of 
clay,  with  bits  of  flint  which  had  formed  part  of  the  original 
rock,  was  left  upon  the  surface.  These  insoluble  grains  and 
nodules,  worked  over  by  the  currents  of  the  rivers,  and  the  sea 
of  the  ensuing  period,  contributed  to  the  formation  of  the 
basal  part  of  the  Pennsylvanian  system. 

Marine  conditions  in  the  West.  —  In  the  Southwest,  where 
changes  of  geography  had  been  slight,  the  interior  sea  had 
been  maintained.  Early  in  the  Pennsylvanian  it  extended 
itself  over  a  much  larger  part  of  the  West.  In  its  clear  waters 
limestone  was  deposited  in  Nevada,  while  shales  and  sand- 
stones are  found  in  Arizona  and  Montana. 

Corals,  crinoids,  and  other  marine  invertebrates  (Figs.  396 
and  397)  flourished  in  these  waters,  as  in  the  preceding  age, 
although  the  number  of  fossils  which  have  been  found  is  far 

376 


THE  PENNSYLVANIAN  PERIOD 


377 


FIG.  396.  —  A  brachiopod 
(Spirifer)  from  the  Penn- 
sylvanian  limestone  of  Col- 
orado. 


less.     We  are  not  to  suppose,  however,  that  the  West  was 

entirely  submerged  at  this  time.     Reddish  sandstones  in  the 

Black  Hills  of  South  Dakota  and  coarse  red  conglomerates 

in  parts  of  Colorado  were  probably 

deposited    upon    land    by    streams. 

They    contain    no    marine    shells. 

These   red    strata   are   linked  with 

the  more  widespread  red  beds  of  the 

Permian  period  and  with  the  peculiar 

conditions  of   its  climate,  —  a  topic 

discussed  in  later  pages. 

Transformation  eastward. — In  the 

Rocky    Mountains    the     Paleozoic 

rocks  have  been  exposed  by  the  upturning  of  the  beds  (Fig. 

450).     Traced  eastward,  they  dip  beneath  Mesozoic  strata 

which  underlie  the  Great  Plains,  reappearing  hundreds  of 
miles  away  in  eastern  Nebraska,  Kan- 
sas, and  Oklahoma.  Where  they  re- 
appear the  Pennsylvanian  system  is 
notably  changed.  Marine  limestones 
are  subordinate,  and  are  interbedded 
with  shales,  sandstones,  and  beds  of 
coal.  Still  farther  east  the  coal  be- 
comes more  abundant  and  the  marine 
Pennsylvanian  strata  correspondingly  less  conspicu- 
ous. Evidently  the  eastern  part  of  the 

country  was  not  the  site  of  a  clear,  open  sea. 

Coal  measures  of  the  East.  —  The  Pennsylvanian  rocks  of 

eastern  United  States  contain  so  many  beds  of  coal  that  they 

are  often  called  the  Coal  Measures.     Only  a  small  part  (about 

2  per  cent)  of  the 

total  thickness  of 

the  system   actu- 
ally consists  of 

coal.     The  section 

(Fig.   398)    shows 


FIG.  397.  —  A  large  spiny 
brachiopod  (Productus) 
of  the 
period. 


FIG.  398.  —  Cross  section  of  Coal  Measures, 
heavy  black  lines  represent  coal  seams. 


Tho 


378 


HISTORICAL  GEOLOGY 


60 


\ 


\ 


that  the  coal  seams,  which  average  but  a  few  feet  in  thickness, 
are  interbedded  with  thicker  laj^ers  of  sandstone,  shale,  and 
other  rocks.  The  details  of  this  section  hold  good  for  a 
single  locality  only.  Elsewhere  we  may  find  fewer  or  more 
coal  seams,  and  the  thicknesses  of  the  individual  beds  vary 

from  place  to  place. 
But  the  general  re- 
lations are  typical  of 
the  whole  region. 

Origin  of  coal.  — 
There  is  ample  proof 
that  coal  is  composed 
of  vegetable  matter 
much  altered  from 
its  original  condition. 
Stumps  of  trees  are 
sometimes  found 
standing  in  the  coal 
seams  as  they  grew; 
delicate  leaves  are 
matted  upon  the 
shales  which  accom- 
pany the  coal;  and  it 
is  often  possible  to 
identify,  even  with 
the  naked  eye,  the 
cellular  structure  of 
plant  tissues  in  pieces 
of  the  pure  coal  itself. 

Vegetable  substance  is  composed  chiefly  of  carbon,  hydro- 
gen, and  oxygen  in  very  complex  compounds.  When  wood 
decays,  chemical  changes  take  place  and  new  substances  are 
produced.  If  this  decay  goes  on  in  the  open  air,  the  carbon, 
hydrogen,  and  oxygen  (most  of  this  from  the  air)  unite  in  such 
a  way  as  to  form  water  and  the  gas  carbon  dioxide.  As  these 
are  volatile,  the  entire  substance  of  the  plant  soon  disappears. 


Wood 


rtsh 


Peat 


.ignite 


Soft 
Coal 


Hard 
Coal 


Graphite 


40 


20 


FIG.  399.  —  Curves  showing  the  changes  which 
take  place  in  the  alteration  of  wood  through 
coal  to  graphite. 

Why  so  little  change  in  the  ash?  What 
proportion  of  the  changes  may  be  passed 
through  while  the  marsh  is  still  unburied  ? 


THE  PENNSYLVANIAN  PERIOD  379 

If,  however,  the  tissues  decompose  under  water,  where  the  air 
is  excluded,  the  changes  are  quite  different  (Fig.  399).  There 
is  not  enough  oxygen  present  to  form  much  carbon  dioxide 
and  water.  The  principal  products  are  a  carbon-hydrogen 
gas,  known  as  marsh  gas,  and  other  compounds  which  contain 
less  carbon.  While  the  bulk  of  the  hydrogen  and  oxygen 
are  thus  removed,  the  carbon  is  only  moderately  reduced, 
and  thus  it  comes  to  form  a  proportionately  larger  part  of  the 
solid  mass  which  is  left.  The  result  of  this  process  is  coal. 
(See  curves  in  Fig.  399.) 

For  the  formation  of  coal,  then,  two  things  are  needed: 
abundant  vegetation,  and  decay  under  water.  In  forests 
we  have  the  first  condition,  but  not  the  second.  In  the  sea, 
decay  takes  place  under 
water,  but  the  vegetation 
is  not  usually  deposited  in 
great  quantity.  In  swamps 
and  marshes,  however,  both 
conditions  are  favorable. 
That  coal  has  actually  come 
from  marsh  deposits  is 
plainly  indicated  by  many 

facts.  The  Seams  of  COal  FiG.  400.  —  Petrified  stump  and  roots 
are  basin-shaped,  being  of  a  tre(;  uncovered  in  a  coal  mine  in 
,,  .  ,  .,*  Scotland. 

thickest  near  the  middle, 

and  thinning  out  into  mere  black  soils  at  the  edges ;  this  is 
just  the  shape  of  existing  marshes.  Again,  we  find  the  old 
stumps  and  rootlets  embedded  in  the  clay  beneath  the  coal 
(Fig.  400),  showing  that  the  vegetation  grew  where  the  coal 
now  lies ;  and  remains  of  aquatic  animals  in  the  midst  of  the 
coal  tell  of  the  presence  of  water  while  it  was  being  deposited. 
Coal,  then,  is  nothing  but  the  half-decomposed  vegetable 
matter  of  swamps,  long  buried  by  later  sediments,  compressed 
by  their  weight,  and  converted  into  a  hard  rock. 

Varieties  of  coal.  —  The  varieties  of  coal  mark  stages  in 
the  process  by  which  the  volatile  components  are  gradually 


380  HISTORICAL  GEOLOGY 

lost.  Peat  is  merely  a  compressed  but  spongy  mass  of  car- 
bonized plants,  such  as  we  may  now  find  beneath  swamps. 
Soft  or  bituminous  coal  has  lost  far  more  of  the  gases  and 
liquids  and  is  a  firm  rock.  Anthracite  or  hard  coal  is  nearly 
all  (91  to  95  per  cent)  carbon  —  a  hard  rock  bearing  little  trace 
of  its  origin. 

The  loss  of  the  volatile  parts  of  the  coal  is  a  very  slow  pro- 
cess. Thus  we  find  that  the  marsh  deposits  of  more  recent 
periods  are  but  partly  converted  into  coal,  while,  at  the 
other  extreme,  the  most  ancient  beds  are  reduced  to  impure 
carbon  alone,  in  the  form  of  graphite.  It  is  not,  however, 
altogether  a  matter  of  age.  In  some  places,  as  in  Colorado, 
igneous  intrusions  have  baked  the  soft  coal 1  into  anthracite, 
or  even  coke.  Wherever  the  coal-bearing  strata  have  been 
strongly  folded,  the  coal  is  found  to  be  much  harder  than  in 
strata  of  the  same  age  where  they  have  not  been  folded.  Thus 
the  Coal  Measures  of  eastern  Pennsylvania  contain  anthracite 
because  the  strata  were  crumpled,  while  in  the  western  part 
of  the  state  they  are  flat  and  afford  only  soft  (or  bituminous) 
coal,  the  age  of  the  rocks  being  the  same  in  both  localities. 

Coal  resources  of  the  United  States.  —  The  wonderful 
development  of  manufacturing  in  the  United  States  is  due  in 
no  small  degree  to  the  presence  of  great  coal  beds  in  the  popu- 
lous eastern  states.  No  other  country,  except  perhaps  China, 
is  so  well  provided  with  this  essential  resource  (Fig.  401). 

Marshy  plain  in  the  East.  —  Returning  to  our  picture  of 
the  United  States  in  Pennsylvanian  times,  we  may  think  of 
the  eastern  part  of  the  country  as  generally  low  and  monoto- 
nously flat.  Vast  swamps  probably  bordered  the  sea  which 
lay  to  the  west,  as  they  now  fringe  the  coast  of  New  Jersey, 
the  Carolinas,  and  Florida.  On  the  east  they  were  flanked 
by  the  land  mass  of  Appalachia.  Inland,  along  the  sluggish 
rivers,  fresh-water  marshes,  like  those  of  the  Mississippi  and 
the  Yukon,  probabty  covered  large  areas.  Gradual  but  halting 
submergence  of  the  region  seems  to  have  been  in  progress. 

1  These  particular  coal  deposits  are  of  much  later  age. 


THE  PENNSYLVANIAN  PERIOD 


381 


Now  and  again  the  swamps  were  inundated,  allowing  sand, 
mud,  and  even  lime  ooze  to  be  spread  over  them.  If  the 
movement  soon  ceased,  the  sea  bottom  was  gradually  built 
up  by  the  sediments  until  the  water  again  became  so  shallow  as 
to  favor  the  growth  of  marsh  plants.  That  the  sea  was  not 


COAU  FIELDS 

OF 
EASTERN  UNITED  STATES 

SCALE  OF  MILES 


FIG.  401.  —  The  coal  fields  of  eastern  United  States.  Those  east  of  the 
Great  Plains  are  largely  of  Pennsylvanian  age,  but  in  part  Triassic  and 
younger.  (U.S.  Geol.  Sure.) 

always  encroaching  on  the  land  is  shown  by  the  presence  of 
unconformities  in  the  Coal  Measures.  While  some  of  these 
are  due  merely  to  the  shifting  of  stream  channels  traversing 
the  marshes,  others  imply  temporary  land  conditions  during 
which  the  rocks  were  eroded  slightly.  In  short,  the  land 
was  very  near  sea  level,  but  was  sometimes  above  it  and 

B.  &  B.  GEOL. 22 


382  HISTORICAL  GEOLOGY 

sometimes  below.  During  the  Pennsylvanian  period  many 
hundreds  of  feet  of  strata  with  many  distinct  coal  beds  accu- 
mulated. 

Coal  Measures  in  Europe.  —  The  marshy  plains  were 
duplicated  on  a  smaller  scate  in  western  Europe ;  and,  from 
the  coal  seams  there  formed,  England,  Germany,  and  adjacent 


FIG.   402.  —  Little  wheatlike  shells  of  protozoans  (Fusulina)  in  a  Pennsyl- 
vanian limestone. 

countries  now  derive  most  of  their  supply  of  coal.  Russia, 
the  Mediterranean  region,  and  southern  Asia,  however,  were 
occupied  by  clear,  open  seas  in  which  thick  beds  of  limestone 
were  deposited.  In  some  places  these  strata  are  so  full  of 
the  little  wheatlike  shells  of  the  protozoan  Fusulina  (Fig.  402) 
that  they  are  generally  known  to  geologists  as  the  "  Fusulina 
limestone." 

LIFE  OF  THE  COAL  SWAMPS 

Plants  well  recorded.  —  Plants  are  known  to  have  been 
plentiful  in  the  Devonian,  and  there  is  reason  to  believe  that 
they  clothed  the  land  surfaces  even  in  much  earlier  periods; 
but  by  the  accident  of  having  large  coal  beds  preserved,  we 
have  in  the  Pennsylvanian  rocks  for  the  first  time  a  satisfac- 
tory record  of  the  plants  of  the  land. 

Dominance  of  the  fernlike  forms.  —  As  in  our  modern 
swamps,  so  in  those  of  the  Pennsylvanian  period,  plants  of  all 
sizes  lived  together  in  the  wet  places.  Little  floating  algae, 
hardly  visible  to  the  eye,  low  sedgelike  forms,  and  even  large 
trees  were  present.  But  there  was  this  difference  :  the  plants 
belonged  more  largely  to  the  lower  branches  of  the  vegetable 
kingdom.  Neither  the  cypress  nor  the  mangrove,  nor  even  the 


THE  PENNSYLVANIAN   PERIOD 


383 


FIG.  403.  —  Leaflet  of 
a  seed  fern  from  the 
Coal  Measures  of 
Pennsylvania. 


tamarack  and  rushes,  were  in  existence  then.  The  prevalent 
plants  were  the  seed  ferns  (Fig.  403)  with  some  true  ferns  and 
other  pteridophytes  (see  p.  293).  They 
were  free  to  occupy  all  the  stations  in 
life  now  held  by  the  higher  seed  plants. 
Some  were  low  herbs,  like  our  modern 
ferns,  while  many  had  developed  woody 
trunks  with  bark,  and  these  rivaled  our 
present  day  trees  in  stature.  The  nu- 
merous stumps  and  fallen  logs  which 
have  been  found  embedded  in  the  coal 
show  that  extensive  forests  of  these  trees 
(Fig.  404)  were  common  in  both  the 
United  States  and  Europe,  as  well  as  in 
the  tropics.  The  graceful  fronds  which 
crowned  the  palmlike  trees  may  often 
be  found  matted  between  layers  of  shale, 
where  they  have  been  preserved  as  in  a 
botanist's  press.  We  can  gain  a  fair  idea  of  the  aspect  of 
the  Carboniferous  forests  by  comparing  the  tree  ferns  which 
still  inhabit  New  Zealand  and  Australia. 

With  the  seed  ferns  were  mingled  dense  thickets  of  reeds, 
resembling  our  familiar  horsetail  grass  (Equisetum).  Many 
reached  the  size  and  perhaps  the  strength  of  the  tall  bamboo 
of  Asia,  although  their  modern  descendants  are  of  lowly 
stature. 

Probably  the  largest  trees  of  the  period  were  the  so-called 
"  scale  trees  "  (Lepidodendron  and  others).  Unlike  the  pre- 
ceding forms,  the  trunks  branched  as  in  our  familiar  elms, 
and  instead  of  broad,  feathery  fronds  their^eaves  were  short 
and  stiff,  and  were  attached  closely  to  the  trunk  and  branches. 
The  nearest  living  relative  of  the  Lepidodendron  is  $i,e  trailing 
club  moss  (which  is  not  a  true  moss  at  all),  —  one  of  the  frailest 
little  herbs  of  our  modern  forests. 

Higher  plants  appear.  —  Thus  far  all  the  coal  plants  which 
have  been  mentioned  have  been  members  of  the  Pteridophyte 


384 


HISTORICAL  GEOLOGY 


group  or  of  that  transitional  class  which  we  have  called  the 
seed  ferns  (p.  293).  There  is  still  no  evidence  of  the  existence 
of  the  plants  with  incased  seeds  and  prominent  flowers,  but 


FIG.  404.  —  Ideal  view  of  the  trees  in  a  Carboniferous  swamp.  The  large 
cone-bearing  tree  in  the  center  is  the  scale  tree  (Lepidodendron).  On  the 
right  are  gigantic  horsetail  reeds  and  on  the  left  Cordaites,  one  of  the  ear- 
liest Gymnosperms.  (After  Horsfall.) 


THE  PENNSYLVANIAN  PERIOD 


385 


FIG.  405.  —  A  small  brachi- 
opod  (Chonetes)  common 
in  the  later  Paleozoic 
rocks. 


the  gymnospcrms  were  represented  in  the  Pennsylvanian 
forests  by  Cordaites,  a  tree  which  com- 
bined many  of  the  characteristics  of 
the  conifers  and  the  palmlike  cycads. 
They  had  long  sword-shaped  leaves 
and  appear  to  belong  to  a  distinctly 
higher  level  of  development  than  any 
of  the  fernlike  plants. 

Land  animals  diversified. — In  older 
systems  of  rocks  the  remains  of  land 

animals  have  been  found  only  rarely.     In 
the  Pennsylvanian,  however,  with  its  well- 
preserved  plants,  the  finding  of  many  air- 
breathing    animals 
should  be  expected. 

Among  the  arthro- 
pods, a  variety  of 
insects  (Fig.  408), 
scorpions,  centipedes, 
and  spiders  testifies  to  the  wide  diver- 
sification through  which  the  group  had 
passed  in  the  periods  before.  As  yet, 
however,  the  bees,  butterflies,  and  other 
highly  specialized  insects  had  not  ap- 
peared, —  a  fact  which  gains  added  in- 


FIG.  406.  —  A  small 
brachiopod  (Pug- 
nax)  with  sharply 
folded  shell. 


FIG. 


407.  —  A  Pennsyl- 
vanian scallop  shell 
( Aviculopecten) . 


terest  from  the  reflection  that 
the  flowers  on  which  these 
animals  now  depend  were  like- 
wise yet  to  come. 

Amphibians  take  the  lead. 
— In  the  Mississippian  period 
amphibians  are  known  to  have 
been  present.  Far  more  abun- 
dant remains  of  them  are 
found  in  the  coal-bearing  rocks  which  followed.  Nearly  all 
appear  to  have  been  like  the  salamanders  in  form,  but  many 


FIG.    408.  —  A   large   winged    insect 
from  the  Coal  Measures  of  France. 


386 


HISTORICAL  GEOLOOY 


of  them  had  more  substantial  bony  frames  and  were  of  larger 
size  (Fig.  409).  Certain  degenerate  types  had  lost  the  use  of 
the  limbs  and  doubtless  adopted  the  habits  of  snakes. 


FIG.  409.  —  A  large  amphibian  of  crocodile-like  form  and  habits,  as  it  prob- 
ably appeared  in  life. 

i '     '(..•-,       ' 

First  appearance  of  the  reptiles. — 'Recently  the  bones  of 
true  reptiles  have  been  discovered  in  the  Coal  Measures  of 
Illinois  and  Pennsylvania.  Not  until  the  next  period,  how- 
ever, does  the  class  come  into  prominence,  and  so  the  discus- 
sion of  them  is  deferred  until  tliat  Chapter  is  reached. 

Climate  of  the  Pennsylvania!!.  —  The  abundance  of  vege- 
tation in  the  coal  swamps  has  been  thought  to  indicate  that 
North  America  and  Europe  were  covered  with  tropical  jungles 
in  which''l!he' growth'  of  plants  was  luxuriantly  rapid.  This 
would  imply  a  climate  warmer  ithan  that  of  the  present,  and 
perhaps  m6ister. 

By  others, 'however,  it  is- pointed  out  that  the  largest  accu- 
mulations 6:f 'peat  are  now  being  formed  in  cool  regions,  such 
as  Canada '  and  -northern  Europe.  'Singularly  enough,  the 
microscopic  -structure  of  the  leaves  of  the  coal  trees  is  much 
more  like  that  o"f  our  northern  conifers  and  other  hardy  plants 
than  like'  the-  clel&aie' and;  iJiih-skihned  leaves  prevalent  in 


THE  PENNSYLVANIAN  PERIOD 


387 


tropical  jungles.  Thick  bark  is  another  feature  shared  by  the 
coal  trees  and  those  of  our  cooler  countries  to-day.  It  seems 
not  improbable,  therefore,  that  the  climate  under  which 
many  of  the  coal  marshes  nourished  was  more  like  that  of 
Canada  than  of  Florida  or  the  Amazon. 

Great  length  of  the  period.  —  While  it  is  not  possible  to 
calculate  exactly  the  duration  of  geologic  periods,  some 
rough  estimates  made  for  the  Pennsylvanian  are  of  interest. 
For  the  growth  of  the  vegetation  which  made  the  coal  seams 
in  a  single  locality  1,000,000  to  2,500,000  years  would  seem 
to  be  required,  and,  for  the  sediments  in  which  they  are  inter- 
bedded,  at  least  as  much  more.  It  is  therefore  possible  that 
5,000,000  years  were  included  in  this  single  period. 

QUESTIONS 

1.  In  Pennsylvania  there  are  thick  beds  of  conglomerate  and 
sandstone  which  contain  no  fossil  shells,  but  an  abundance  of  plant 
leaves  in  certain  layers.     Can  you  suggest  the  origin  of  these  rocks  ? 

2.  The  sandstone  which 

lies  at  the  base  of  the  Coal  ^^^^^^::::. Sandstone :•.- 
Measures  in  many  parts  of 
this  country  and  Europe  is 
sometimes  called  the  mill- 
stone grit.  Can  you  suggest 
why  this  name  was  given  it  ? 


FIG.  410.  —  A  sandstone  "cut  out"  in  a 
coal  seam. 


3.  The  accompanying  cross  section  (Fig.  410)  shows  a  narrow, 
winding  bed  of  sandstone  lying  in  a  coal  seam. 
Such  things  are  known  to  the  miners  as  "cut 
outs."  How  may  such  a  feature  have  been 
produced  ? 

4.  Tell  all  the  events  and  changes  which 
you  find  recorded  by  the  accompanying  sec- 
tion taken  from  the  Coal  Measures  of  Ohio 
(Fig.  411). 

5.  Coal  seams  are  often  broken  by  faults. 
411.  —  Section    Jf  in  flowing  a  certain  coal  seam  in  a  mine, 

you  should  encounter  such  a  fault,  how  could 
you  tell  whether  to  hunt  for  the  lost  continua- 
tion of  the  bed  at  a  higher  or  lower  level  ? 

6.   The  Pennsylvanian  strata  are  the  latest  widespread  Paleozoic 


FIG. 
from      Ohio 
Measures. 


Coal 


388  HISTORICAL  GEOLOGY 

deposits  in  eastern  United  States.  Why  should  their  outcrops,  as 
shown  in  the  map  (Fig.  401),  be  so  different  in  shape  from  those 
of  the  Cambrian  system? 

7.  In  Rhode  Island  the  coal  is  very  hard  and  graphitic.      From 
this  fact  what  do  you  suspect  with  reference  to  the  structure  of 
the  Coal  Measures  in  that  district? 

8.  From  which  variety  of  coal  could  illuminating  gas  be  made 
to  the  best  advantage,  and  why  ? 

9.  Of  the  mineral  products  which  you  have  studied  thus  far, 
which  most  resembles  coal  in  method  of  occurrence,  —  iron,  copper, 
or  zinc  ? 

10.  Iron  ore  is  not  infrequently  deposited  in  bogs  at  the  present 
time.     That  being  true,  in  what  part  of  the  United  States  might 
such  ore  be  expected  in  formations  of  Pennsylvanian  age  ? 


CHAPTER  XIX 
THE   PERMIAN   PERIOD 

A  transition  period.  —  The  Permian  marks  the  transition 
from  the  Paleozoic  era  to  the  Mesozoic.  In  eastern  United 
States  the  Permian  rocks  are  a  mere  continuation  of  the  Penn- 
sylvanian  system,  while  in  the  Rocky  Mountains  it  is  often 
impossible  to  separate  Permian  strata  from  those  of  the  Trias- 
sic  period.  Only  locally  are  the  systems  sharply  marked  off 
from  each  other. 

Emergence  of  the  eastern  region.  --Throughout  the  Per- 
mian, the  interior  sea  was  slowly  being  withdrawn.  The 
deposition  of  the  Coal  Measures  continued  on  into  the  early 
Permian  in  the  Ohio  Valley.  Some  of  the  sediments  were 
laid  down  in  rivers  or  fresh-water  lakes,  and  contain  abundant 
leaves  of  plants.  Later  in  the  period  the  region  seems  to 
have  been  drained,  leaving  a  broad  lowland  which  was  only 
feebly  eroded. 

The  sea  lingers  in  the  Southwest.  —  In  the  southern  part 
of  the  tract  which  we  now  call  the  Great  Plains  the  sea 
lingered  somewhat  later.  Shales,  sandstones,  and  limestones 
quietly  accumulated  in  Texas,  and  perhaps  in  Kansas. 

The  red  beds.  —  The  later  Permian  rocks  of  northern 
Texas,  however,  tell  of  very  different  conditions;  they  are 
reddish  shales,  with  layers  of  gypsum  and  rock  salt.  Evi- 
dently the  sea  had  by  that  time  receded  still  farther,  leaving 
a  desert  region  in  western  United  States,  with  saline  lakes 
in  the  depressions.  In  Colorado  and  some  other  places  the 
formation  of  these  red  beds  began  during  the  Pennsylvanian 
and  continued  on  into  the  Triassic  period,  indicating  that  the 
arid  climate  was  of  long  duration. 

389 


390  HISTORICAL  GEOLOGY 

Expansion  of  Permian  lands.  —  The  gradual  withdrawal 
of  the  epicontinental  sea  eventually  left  almost  all  of  the 
continental  platform  dry  land.  On  studying  other  countries 
we  find  evidence  that  there,  likewise,  the  land  was  more 
extensive  than  at  any  other  period  in  the  Paleozoic  era.  The 
Permian  was  everywhere  a  time  of  expanded  continents. 
To  explain  such  a  general  withdrawal  of  the  seas  the  sugges- 
tion has  been  made  that  the  deep  ocean  basins  sank  slightly, 
thus  leaving  the  continents  relatively  higher  than  before. 
So  widespread  and  "radical  a  change  marks  this  as  one  of  the 
critical  periods  of  geologic  history. 

The  Appalachian  trough.  --  Throughout  the  Paleozoic  era, 
the  thickest  layers  of  sediments  had  been  laid  down  in  the 
interior  sea  just  west  of  the  old  Appalachian  land,  from  New 
England  to  Alabama,  and  even  across  to  Oklahoma.  This 
curved  belt  had  been  a  subsiding  trough,  sinking  perhaps 
because  of  the  weight  of  sediments  which  were  constantly 
loaded  upon  it.  Much  of  the  time  the  sinking  just  kept  pace 
with  the  deposition  of  sediment,  so  that  thousands  of  feet  of 
strata  were  formed  in  relatively  shallow  sea  water,  as  we  now 
learn  from  the  presence  of  such  things  as  .coral  reefs.  At 
other  times,  the  subsidence  was  more  rapid,  and  deeper  water 
prevailed,  or  on  the  other  hand  was  of  a  halting  nature  and 
allowed  the  coastal  rivers  to  build  out  the  seashore  with  allu- 
vial deposits.  Finally,  in  the  Permian,  the  sinking  and  the 
sedimentation  ceased,  and  the  process  was  reversed  into  a 
slow  emergence.  The  sediments  deposited  in  this  trough  had 
then  reached  a  thickness  much  greater  than  that  of  the 
corresponding  strata  in  the  Mississippi  Basin. 

Crumpling  of  the  east  flank  of  the  continent.  Near  the 
close  of  the  Permian,  whether  as  a  result  of  a  sinking  in  the 
Atlantic  basin,  or  from  some  other  cause,  the 'east  side  of 
North  America  was  subjected  to  powerful  horizontal  com- 
pression. The  Appalachian  trough  was  a  weak  zone  'in  the 
crust,  just  as  the  bend  in  a  crooked  stick  determines  the 
point  at  which  it  will  break  when  pressure  is  applied  at 


THE  PERMIAN   PERIOD 


391 


the  ends.  From  Newfoundland  to  Alabama  and  even  into 
Oklahoma  the  rocks  were  thus  crumpled.  The  rocks  of  the 
old  Appalachian  land  had  been  folded  more  than  once  before, 
and  so  the  Permian  deformation  added  little  to  the  com- 


FIG.  412.  —  An  ideal  representation  of  the  west  coast  of  Appalachia,  during 
the  Paleozoic  era.  The  ancient  rocks  on  the  east  are  being  eroded,  and 
the  sediments  laid  down  in  the  sea  gradually  become  finer  westward* 
(Modified  after  Willis.) 

plexity  of  their  structure.  The  Paleozoic  rocks  in  the  great 
trough,  however,  were  now  folded  for  the  first  time  (Figs. 
412  and  413).  In  Pennsylvania,  the  strata  were  bent  into  a 
series  of  open  anticlines  and  synclines  (Fig.  46).  Farther 


FIG.  413.  —  The  same,  after  folding  of  the  Paleozoic  sediments.  The 
unshaded  portion  shows  the  folds  restored  as  they  might  have  been  if 
they  had  not  been  affected  by  erosion.  (Modified  after  Willis.) 

south,  where  deformation  was  greater,  the  folds  were  com- 
pressed and  overturned  westward  (Fig.  47).  Here  and  there 
the  stiff  limestone  and  quartzite  formations  were  broken 
and  thrust  over  the  adjacent  rocks  (Fig.  414).  At  the  same 


FIG.  414.  —  Closely  folded  strata  in  the  southern  part  of  the  Appalachian 
mountains.     (U.S.  Geol.  Surv.) 

time  the  soft  shales  were  crumpled  and  crushed  beneath  the 
stronger  beds.  So  numerous  are  these  overthrusts  in  some 
districts  that  the  original  folds  cannot  now  be  reconstructed. 


392  HISTORICAL  GEOLOGY 

Folding  a  slow  process.  —  It  is  probable  that  the  folding 
was  accomplished  very  slowly,  as  are  the  larger  earth  move- 
ments of  the  present  time.  The  anticlines  probably  rose  so 
gradually  that  the  decay  of  the  rocks  and  the  work  of  streams 
partially  kept  pace  with  the  growth,  so  that  the  young  moun- 
tains were  at  all  times  ragged  and  gashed  with  valleys.  This 
is  true  of  growing  mountains  to-day,  such  as  the  Coast  Range 
of  California  and  the  St.  Elias  chain  in  Alaska,  and  it  is  safe 
to  judge  the  Permian  by  the  present.  Even  the  great  thrust 
faults,  along  which  massive  limestones  have  been  pushed 
several  miles  over  younger  beds,  were  doubtless  made  by  a 
succession  of  slippings,  each  advancing  not  more  than  a  few 
feet,  and  each  slip  separated  from  the  next  one  perhaps  by 
months  or  years  of  time.  While  recognizing,  however,  that 
the  folding  and  the  uplift  were  very  slow,  we  may  well  imagine 
the  first  Appalachian  and  Ouachita  (Oklahoma)  Mountains 
as  a  series  of  lofty,  rugged  ranges  comparable  to  the  Alps,  or 
to  the  modern  Pacific  ranges  in  North  America. 

Effects  of  expanding  the  continents.  --The  widespread 
emergence  of  the  continents  into  dry  land  must  have  produced 
important  changes,  not  only  in  the  geography  of  the  period, 
but  in  the  conditions  of  life  for  plants  and  animals,  and  even 
in  the  condition  of  the  atmosphere  and  in  the  climate.  It  will 
be  best  to  inquire  into  these  matters  separately. 

Adversities  inflicted  on  the  sea  life.  —  During  the  Paleo- 
zoic era  the  shallow  epicontinental  seas  had  harbored  abun- 
dant marine  animals  and  plants.  With  the  exclusion  of  these 
broad  sheets  of  water  from  the  continents  the  home  of  such 
forms  of  life  was  much  reduced  in  size.  It  would  be  entirely 
natural  to  expect  that  as  a  result  the  competition  for  a  living 
would  become  much  keener,  and  that  many  of  the  forms  less 
able  to  adapt  themselves  to  the  new  conditions  would  be 
exterminated.  This  may  be  the  explanation  of  the  well- 
known  fact  that  very  few  of  the  distinctly  Paleozoic  fossils 
pass  on  into  the  Triassic  system.  Few  of  the  large  groups 
entirely  disappeared,  although  some  were  much  diminished, 


THE  PERMIAN  PERIOD  393 

while  others,  such  as  the  corals,  were  largely  reorganized  on 
new  plans.  The  change  is  seemingly  one  of  the  most  abrupt 
and  profound  in  all  the  geologic  record.  Yet  in  northern 
India  and  California,  where  the  Permian  seas  lingered  on 
into  the  Triassic,  the  change  in  the  fossils  is  gradual  and  no 
sharp  dividing  line  can  be  drawn. 

More  ample  opportunities  for  the  land  life.  —  The  very 
changes  which  restricted  the  habitation  of  the  corals,  mollusks, 
and  their  kin  gave  wider  room  to  the  denizens  of  the  land.  A 
great  abundance  and  variety  of  plants  and  insects  were  already 
present.  The  salamanderlike  types  of  amphibians  were 
even  more  numerous  and  better  constructed  than  in  the 
Pennsylvanian.  In  fact,  they  were  never  afterward  as  promi- 
nent as  at  this  time.  Amphibians  nowadays  are  small  crea- 
tures, and  most  of  them  have  soft  bodies;  but  some  of  the 
Permian  types  were  large  and  were  more  comparable  to  rep- 
tiles like  the  crocodiles  of  to-day  with  their  bony-plated  heads, 
powerful  muscles,  and  formidable  array  of  teeth. 

Reptiles  gain  the  ascendancy.  —  It  was  left  for  the  true 
reptiles,  however,  to  gain  supremacy  among  the  land  animals 
in  the  Permian  period,  in  spite  of  their  amphibian  rivals. 
When  any  group  first  appears  in  geologic  history,  it  is  apt  to 
be 'represented  by  closely  related  kinds  unlike  those  which  live 
to-day.  These  are  known  as  generalized  types,  because  they 
combine  vaguely  in  one  animal  the  characteristics  of  several 
later  kinds.  Thus  in  the  Permian  there  was  one  kind  of  rep- 
tile which  resembled  in  some  respects  the  crocodiles,  the 
lizards,  and  other  types  now  extinct,  and  yet  cannot  be 
classed  with  any  one  of  these  groups  more  than  with  the  others. 
Later,  these  generalized  types  branched  out  into  the  distinct 
forms  now  recognized.  These  will  be  described  in  connection 
with  a  later  period. 

Prevalence  of  arid  climates.  —  The  abundance  of  salt  and 
gypsum  beds  in  the  Permian  strata  of  many  countries  has 
already  been  mentioned  as  indicative  of  desert  conditions. 
Deserts  are,  of  course,  only  possible  on  land,  and  to-day  they 


394  HISTORICAL  GEOLOGY 

are  especially  well  developed  in  the  interior  regions  of  large 
continents,  for  example,  in  central  Asia.  Wide  extension  of 
lands,  as  in  the  Permian,  therefore  favors  the  making  of  deserts 
in  appropriate  places. 

Glacial  conditions  in  the  tropics.  —  While  considering  the 
Permian  climate  we  must  not  fail  to  note  what  is  easily  the 
most  remarkable  fact  now  known  about  the  period.  Asso- 
ciated with  rocks  of  Permian  age  layers  of  glacial  till  have  been 
found  lying  upon  scratched  and  grooved  surfaces  of  older 
rocks.  These  glacial  beds  have  not  been  discovered  in  the 
polar  regions,  as  we  might  confidently  expect,  but  in  India, 
South  Africa,  South  America,  and  Australia ;  that  is  to  say, 
near  and  even  within  the  tropics.  Nor  are  we  to  suppose  that 
glaciers  were  confined  to  lofty  mountains.  The  limestones 
and  shales  with  which  some  of  the  layers  of  till  are  associated 
show  that  they  were  deposited  near  or  even  below  sea  level. 

The  existence  of  glaciers  over  so  wide  an  expanse  of  the 
earth's  surface  and  even  within  the  tropics  themselves  points 
to  most  unusual  climatic  conditions.  We  may  well  believe 
that  they  indicate  a  colder  climate  than  now  over  much  of  the 
globe ;  but  mere  cold  does  not  account  for  the  strange  dis- 
tribution of  the  glaciers,  and  a  satisfactory  explanation  of  all 
the  facts  is  still  lacking. 

SUMMARY  OF  THE  PALEOZOIC  ERA 

Geographic  conditions.  —  From  the  Cambrian  to  the 
Permian,  the  more  persistent  lands  were  in  eastern  Canada 
and  southeastern  United  States.  The  central  and  western 
parts  of  the  continent  were  repeatedly  submerged  by  a  rela- 
tively shallow  sea.  As  stated  on  a  previous  page,  the  copious 
supply  of  detritus  from  the  Appalachian  land  built  up  the 
thickest  Paleozoic  formations  along  the  eastern  border  of  the 
interior  sea,  while  in  the  middle  of  the  continent  sedimenta- 
tion went  on  more  slowly. 

In  the  far  West,  the  Colorado  region  and  parts  of  the  Pacific 


THE  PERMIAN   PERIOD  395 

slope  were  occasionally  out  of  water,  —  although  there  is 
little  evidence  that  they  were  as  mountainous  as  now.  Where 
the  Great  Basin  now  is,  the  sea  was  especially  long-lived, 
and  thousands  of  feet  of  varying  marine  sediments  were 
there  laid  down. 

On  the  whole,  the  seas  overlapped  the  continent  more  in  the 
early  than  in  the  later  part  of  the  era. 

Throughout  the  Paleozoic  periods  volcanic  activity  was 
confined  almost  entirely  to  the  Pacific  side  of  North  America, 
much  as  in  recent  times.  In  Europe,  however,  most  of  the 
volcanoes  were  near  the  Atlantic.  They  were  particularly 
numerous  at  times  in  the  British  Isles.  (How  does  this 
compare  with  modern  conditions?) 

Climatic  conditions.  —  The  climates  of  the  Paleozoic 
periods  have  left  only  a  scanty  record.  That  there  were 
deserts  in  several  countries  at  various  times  is  proven  by 
such  saline  deposits  as  those  of  New  York  and  Michigan; 
and  that  moist  conditions  prevailed  at  other  times  is  indicated 
by  the  coal  beds.  It  was  formerly  supposed  that  the  earth 
was  hotter  in  Paleozoic  times  than  now,  but  Cambrian  and 
Permian  glacial  deposits  in  regions  which  are  now  semi- 
tropical  seem  to  preclude  this  view.  Of  the  distribution  of  the 
climatic  zones  we  know  almost  nothing.  We  can  infer  their 
existence  only  because  as  long  as  the  earth  is  round,  and  the 
surface  part  receives  its  heat  from  the  sun,  such  zones  must 
be  present. 

Development  of  plants  and  animals.  —  In  reviewing 
the  progress  of  the  living  things  we  see  many  noteworthy 
changes  in  the  course  of  the  Paleozoic  era.  The  early  periods 
were  dominated  by  the  invertebrated  animals.  One  by  one, 
new  groups  made  their  appearance ;  and  of  these,  some,  like 
the  trilobites,  passed  their  prime  and  slowly  dwindled  to  extinc- 
tion, while  others  merely  retired  to  a  subordinate  position  in 
the  scale  of  life. 

In  the  later  periods,  fishes  and  amphibians  successively 
came  to  the  front,'  proved  themselves  more  powerful  than  the 


396 


HISTORICAL  GEOLOGY 


invertebrates  which  preceded  them,  and  in  turn  yielded  par- 
tially to  the  reptiles,  which  began  their  rise  near  the  close  of 
the  era. 

Of  the  evolution  of  the  plants  we  know  much  less ;  but  it  is 
clear  that  the  ferns  and  some  of  the  gymnosperms  were  the 
prevalent  types  in  the  later  Paleozoic  periods.  The  higher 
groups  were  still  to  be  evolved. 

QUESTIONS 

1.  Is  it  necessary  to  assume  that  salt  lakes  are  detached  parts 
of  the  ocean  ?     Can  fresh  lakes  ever  become  salt,  and  if  so,  how  ? 

2.  Can  you  suggest  a  reason  why  desert  sandstones  like  some 
of  those  in  the  Permian  system  are  usually  cross  bedded  ? 

3.  In  Germany  a  single  bed  of  salt  in  the  Permian  system  is 
said  to  be  more  than  four  thousand  feet  thick.     What  does  this 
indicate  ? 

4.  What  is  the  significance  of  the  Permian  system  in  India  with 
reference  to  Laplace's  theory  of  the  origin  of  the  earth  ? 

5.  Although  in  Australia 
the  center  of  Permian  gla- 
ciation  is  not  known,  and 
neither  surface  moraines  nor 
drumlins  have  been  identi- 
fied, the  direction  of  glacial 

FIG.  415. -Glacial  markings  on  a  rock     movement   ^s   been  deter- 
surface.  mined.     Can     you     suggest 

how  from  a  single  striated 

surface  of  rock  (such  as  represented  in  Fig.  415)  one  may  learn 
whether  the  ice  was  moving  toward  the  left  or  right  ? 

6.  It  is  often  said  that  coal  has  been  formed  in  tropical  jungles. 
What  is  the  significance  of  the  coal  beds  which  lie  between  sheets 
of  glacial  till  in  Australia  ? 


CHAPTER  XX 
THE    TRIASSIC    PERIOD 

Transition  from  Paleozoic  to  Mesozoic.  —  The  Permian 
period  fades  into  the  Triassic  so  gradually  that  a  dividing 
line  cannot  always  be  drawn  between  them.  In  eastern 
United  States,  it  is  true,  great  geographic  changes  had  taken 
place,  and  the  crumpling  of  the  Appalachian  sediments  had 
produced  ranges  of  mountains  where  there  had  been  a  coastal 
plain  before ;  but  the  rest  of  the  continent  remained  much 
as  in  the  Permian  period. 

Deserts  continue  in  the  West.  —  In  the  Triassic,  as  in 
the  Permian,  red  sediments  were  deposited  where  the  Great 
Plains  and  Rocky  Mountains  now  stand.  Beds  of  gypsum, 
the  relics  of  salt  lakes,  point  to  the  prevalence  of  desert  condi- 
tions, much  as  in  Nevada  and  Utah  to-day.  Needless  to  say, 
the  remains  of  living  things  are  very  rare  in  the  red  beds.  It 
is  probable  that  much  of  the  West  was  an  inhospitable  place 
for  both  plants  and  animals,  except  such  hardy  forms  as  were 
fitted  to  live  in  an  arid  land.1 

Marine  strata  on  the  Pacific  slope.  —  Marine  rocks  of 
Triassic  age  are  found  only  on  the  western  border  of  the 
continent,  and  in  but  few  places  even  there.  From  this  we 
may  infer  that  the  seas  which  had  been  drawn  off  from  the 
continental  platform  during  the  Permian  were  very  slow  in 
creeping  back  upon  it.  The  Pacific  is  the  only  ocean  known 
to  have  encroached  upon  the  North  American  land  during  the 
period.  Its  shore  line  lay  somewhat  farther  east  than  now, 
especially  in  British  Columbia  and  Alaska.  In  the  United 
States  it  reached  Idaho  and  Nevada.  Off  this  coast  thick 

1  It  may  be  noted,  in  passing,  that  desert  conditions  were  prevalent  also 
in  western  Europe  at  this  time,  as  they  had  been  in  the  Permian  period. 
B.  &  B.  GEOL.— 23  397 


398  HISTORICAL  GEOLOGY 

banks  of  mud  and  ooze  accumulated.  In  consequence  of 
compression  at  a  later  time,  the  sediments  are  now  folded 
slates  and  schists. 

Erosion  of  the  new  Appalachian  Mountains.  —  No  surely 
marine  beds  of  Triassic  age  are  exposed  in  the  eastern  part  of 
the  continent.  The  land  probably  extended  out  even  beyond 
the  present  Atlantic  shore. 

The  growth  of  the  young  Appalachian  Mountains  seems 
not  to  have  ceased  entirely  in  the  Triassic  period.  As  the 
Paleozoic  rocks  were  folded,  the  eroded  surface  of  old  Appa- 
lachia  was  likewise  warped,  and,  in  the  broad  downwarps, 
streams  continually  spread  the  abundant  sand  and  silt  which 
they  were  removing  from  the  mountains.  The  deposits  on 
the  low  slopes  and  bottoms  of  these  basins  eventually  reached 
a  thickness  of  thousands  of  feet ;  and,  as  is  common  among 
rapidly  accumulating  sediments,  the  particles  were  not  well 
assorted.  Alternations  of  sandstone  and  shale  of  varying 
colors  are  characteristic  of  the  Newark  formation,  as  these 
beds  are  called.  Red  is  the  predominating  color. 

Locally,  as  in  southern  Virginia,  the  Triassic  rocks  include 
a  few  beds  of  coal,  which  bespeak  the  growth  of  marshes  in 
the  low  grounds.  The  fact  that  these  marshes  existed  prob- 
ably means  that  the  climate  of  the  Atlantic  seaboard  was  less 
arid  than  that  of  the  West. 

Volcanic  eruptions  in  the  Atlantic  slope.  —  About  the  time 
the  Newark  sediments  were  being  laid  down,  eruptions  of 
basaltic  lava  occurred  in  the  same  district.  Some  of  the 
basalt  sheets  rise  obliquely  through  the  strata,  thus  proving 
that  they  were  squeezed  into  the  rocks  as  intrusions.  Others 
have  cindery  surfaces  and  are  overlain  by  sandstones  which 
are  not  altered  at  the  contact  with  the  lava.  Here,  evidently, 
the  flows  were  poured  out  upon  the  surface  and  afterwards 
buried  beneath  sediments. 

Being  harder  than  the  sedimentary  strata,  the  lava  sheets 
have  been  left  as  ridges  in  the  subsequent  wasting  of  the 
surface.  The  palisades  of  the  Hudson  and  such  heights  as 


THE  TRIASSIC  PERIOD 


399 


Mt.  Holyoke  in  Massachusetts  are  merely  the  outcropping 
edges  of  hard  Triassic  lava  sheets,  formed  during  this  epoch 
of  volcanic  activity,  —  the  last  in  the  history  of  the  eastern 
portion  of  North  America. 


FIG.  416. —  A  Triassic 
brachiopod  (Terebrat- 
ula).  The  majority 
of  Mesozoic  brachio- 
pods  have  this  general 
form. 


LIFE  OF  THE  TRIASSIC 

New  aspect  of  the  marine  invertebrates.  —  Although  the 
lower  animals  of  the  Triassic  seas  resemble  the  Paleozoic 
types  in  many  ways,  the  differences  are 
nevertheless  very  distinct.  Not  only 
had  some  of  the  Paleozoic  groups,  such 
as  the  trilobites,  wholly  disappeared, 
but  others,  as  the  brachiopods  (Fig. 
416),  had  been  relegated  to  an  inferior 
station. 

The  mollusks  became  the  most  abun- 
dant and  conspicuous  of  the  shelled  ani- 
mals, and  among  them  the  group  of 
coiled  cephalopods  had  a  remarkable 
development  during  the  Triassic  and 
later  Mesozoic  periods.  From  the  simple 
types  with  straight  sutures,  they  had 

advanced 
later  in  the 
Paleozoic  to 
the  posses- 
sion of  lobed 

or  wavy  sutures.  In  the  Mesozoic 
era  the  folding  of  the  partitions 
became  most  intricate  (Figs.  418 
and  419),  producing  equally  com- 
^--^_L  -  *  plicated  suture  patterns. 

FIG.  418.  — An  ammonite  with       Reptiles  overreach  the  amphib- 
part  of  the  outer    shell  re-  ians — The    brief    supremacy   of 

moved  to  show  the  complexly  .  ..  .  ,      , 

folded  sutures.  the  clumsy  amphibians  had  now 


FIG.  417.  — A  small  pele- 
cypod  from  the  Triassic 
limestone  of  Europe. 


400 


HISTORICAL  GEOLOGY 


passed.    Large  alligatorlike  forms  of  powerful  build  were  still 
common  in  the  Triassic  marshes,  but  after  this  period  the  class 

was  represented  only  by 
smaller  soft-bodied  types, 
such  as  frogs  and  salaman- 
ders. 

The  reptiles  were  the  class 
in  power.  —  The  bones  of 
many  reptiles  have  been 
found  in  the  Triassic  rocks, 
and  the  existence  of  others  is 
made  known  to  us  by  the 
host  of  footprints  (Fig.  420) 
which  they  left  upon  the 
muddy  flats  along  the  slack 
rivers  and  bays,  as  in  the 
valley  of  the  Connecticut 
River.  The  mud  has  since 
hardened  into  stone,  but  without  effacing  the  footprints. 
Some  of  these  tracks  show  the  marks  of  three  toes,  and  were 
at  first  very  naturally  mistaken  for  those  of  birds. 


FIG.  419.  —  One  of  the  simpler  ammo- 
nites (Ceratites)  showing  the  moder- 
ately folded  sutures. 


FIG.  420.  —  Tracks  of  three-toed  reptiles  found  in  the  Triassic  sandstone  of 
the  Connecticut  Valley.     (After  Hitchcock.) 


THE  TRIASSIC  PERIOD  401 

The  reptiles  adopt  many  roles  of  life. — There  seems  to  be  no 
doubt  that  the  ancestors  of  the  early  reptiles  were  the  amphib- 
ians. We  may  think  of  them,  then,  as  originally  inhabitants 
of  marshes  and  inland  bodies  of  water.  Reptiles  adapted  for 
such  a  partially  aquatic  life  were  rather  common  in  both  the 
Permian  and  Triassic  periods,  as  their  fossil  remains  attest. 

The  terrestrial  type  becomes  prominent.  —  Some  of  the 
reptiles  came  to  spend  more  and  more  time  on  land,  and  even- 
tually became  fitted  for  living  wholly  under  such  conditions. 
Some  walked  on  all  fours,  as  do  our  modern  cattle  and  many 
other  mammals,  but  the  more  agile  varieties  apparently  were 
leapers,  using  their  powerful  hind  legs  and  stout  tails  after 
the  manner  of  the  kangaroo.  The  majority  of  these  swifter 
forms  seem  to  have  preyed  upon  other  animals,  and  for  this 
purpose  their  teeth  were  sharp  and  strong.  They  played  the 
role  of  our  modern  beasts  of  prey,  such  as  the  tiger  and  the 
wolf,  although  in  a  manner  no  doubt  peculiar  to  themselves. 

Reptiles  find  a  place  in  the  sea.  —  The  tempting  source  of 
food  which  the  fishes  and  mollusks  of  the  sea  presented  was 
early  appropriated  by  other  branches  of  the  reptilian  stem. 
Two  main  types  were  represented  in  the  Triassic  period.  Of 
these  the  Plesiosaurs  were  large,  flattened  saurians  with  long, 
slender  necks  and  short  heads.  Their  legs  eventually  became 
mere  paddles  like  those  of  the  modern  sea  turtles.  The  sus- 
picion that  they  fed  partly  upon  mollusks,  for  which  they 
probably  delved  in  the  shallows  with  their  long  necks,  is 
strengthened  by  the  finding  within  their  bodies  of  smooth 
pebbles,  thought  to  be  gizzard-stones  used  to  pulverize  the 
food  swallowed. 

Of  all  the  reptiles  none  were  better  fitted  for  living  ex- 
clusively in  the  open  sea  than  the  Ichthyosaurs  (fish-reptiles). 
They  had  acquired  the  form  of  fishes  themselves  (Fig.  421), 
with  the  long  powerful  tail  fin,  the  short  neck,  and  long  jaws 
set  with  sharp  teeth.  These  animals  seem  to  have  been 
almost  exclusively  fish  eaters.  Most  aquatic  reptiles,  for 
example  the  turtles,  lay  their  eggs  in  sand  along  the  shore; 


402 


HISTORICAL  GEOLOGY 


but  the  ichthyosaur,  having  lost  the  power  of  resorting  to  the 
shore  to  lay  its  eggs,  was  in  the  habit  of  bringing  forth  its 
young  alive,  as  is  proved  by  the  interesting  find  of  five  little 
ichthyosaur  skeletons  undamaged  within  the  skeleton  of  one 
of  these  reptiles. 


FIG.  421.  — A  family  of  fish  reptiles  (Ichthyosaurus).  (Painted  by  C.  R. 
Knight,  under  the  direction  of  Professor  H.  F.  Osborn.  Copyright  by 
Amer.  Mus.  of  Nat.  Hist.) 

Mammals  make  a  feeble  beginning.  —  Were  it  not  for  the 
overwhelming  predominance  to  which  the  mammals  after- 
wards attained,  their  first  appearance  would  be  scarcely  worth 
mentioning.  In  the  Triassic  rocks  a  few  little  bones  have 
been  found  which  seem  to  be  those  of  primitive  mammals. 


THE  TRIASSIC  PERIOD 


403 


They  were  as  small  and  insignificant  as  the  moles  of  to-day. 
The  most  distinguishing  thing  about  them  is  that  their  teeth 
were  differentiated  into  incisors,  canines,  and  molars,  as  in 
mammals,  while  almost  all  reptiles  have  merely  pointed 
teeth  much  alike  throughout  the  jaw.  The  derivation  of 
mammals  from  some  of  the  Permian  land  reptiles  is  now  the 
most  favored  view. 


FIG.  422.  —  A  living  Mexican  cycad.     (Photograph  by  C.  J.  Chamberlain.) 

Conifers  and  cycads  make  the  forests.  —  A  change  in  the 
plants  which  had  been  in  progress  during  the  Permian  was 
clearly  defined  in  the  early  Mesozoic  periods.  The  great 
ferns  and  allied  trees  of  the  Carboniferous  forests  were  sup- 
planted by  a  higher  group  (gymnosperms)  containing  the 
conifers  and  the  palmlike  trees  called  cycads  (Fig.  422). 


404 


HISTORICAL  GEOLOGY 


Ferns  continued  to  be  common,  but  there  were  more  of  the 
small  varieties,  like  those  now  growing  in  our  forests,  than  of 
the  tree  ferns.  The  woodlands  of  the  Triassic  times  doubtless 
had  a  somber  aspect  not  unlike  that  of  our  pine  forests  to-day. 
Nor  is  it  surprising  that  the  purely  vegetarian  land  animals 
were  then  so  little  developed,  when  we  consider  that  the 
tough,  fibrous  leaves  of  palms  and  the  resinous  needles  of  pines 
and  similar  plants  are  among  the  least  palatable  foods  for  our 
modern  cattle  and  wild  animals.  The  introduction  of  these 
higher  animals  seems  to  have  awaited  the  evolution  of  the 
flowering  plants,  particularly  the  grasses. 

QUESTIONS 

1.  In  the  Humboldt  Mountains  of  Nevada  the  marine  Triassic 
rocks  rest  on  metamorphosed  pre-Cambrian  beds.  What  different 
explanations  may  be  offered  ? 


Fio.  423.  —  Conglom- 
erate with  lava  peb- 
bles resting  upon  a 
sheet  of  lava. 


FIG.  424.  —  Sheet  of 
lava  with  secondary 
minerals  in  the  ad- 
jacent shale. 


FIG.  425.  — A  sheet 
of  lava  with  in- 
cluded pieces  of  the 
adjacent  rock. 


FIG.  426.  —  Sheets  of  lava  inter- 
bedded  with  layers  of  tuff. 


2.  In  the  diagrams    (Figs.  423- 
426),   which   are  the  intrusive  and 
which  are  the  extrusive  lava  sheets, 
and  how  are  the  facts  known  ? 

3.  Small  bodies  of  copper  ore  have 
been   found   in  the   Newark   rocks. 
What   do   you   suspect  as   to   their 
origin  ? 

4.  Some  of  the  sandstones  in  the 


Newark  series  contain  abundant  grains  of  feldspar  and  flakes  of  mica. 
How  do  these  rocks  differ  from  ordinary  sandstone  ?  Can  you  suggest 
the  conditions  under  which  the  two  different  varieties  are  made  ? 

5.    What  kinds  of  fossils  would  you  expect  to  find  in  the  Newark 
series,  and  what  about  their  abundance?     Why? 


CHAPTER  XXI 


1 


-^-^^--=--^-~ 


FIG.  427.  —  Triassic  sediments  in  Connect- 
icut with  a  surface  flow  and  intrusions 
of  lava,  as  they  may  be  supposed  to  have 
existed  before  faulting. 


THE    JURASSIC    PERIOD 

Land  in  eastern  North  America.  Rocks  which  were  made 
during  the  Jurassic  period  are  not  extensively  exposed  in  the 
United  States,  and  for  the  most  part  they  are  less  well  known 
than  those  of  other  periods.  In  the  eastern  half  of  the  country 
no  rocks  which  are  surely 1  of  this  age  have  been  discovered. 
From  this  circumstance  it  seems  probable  that  the  eastern 
part  of  the  continent  was 
largely  out  of  water,  and 
that  the  erosion  of  the 
Appalachian  region  was 
still  in  progress.  After 
the  deposition  of  the 
Newark  sediments  during 
the  later  part  of  the  Tri- 
assic period,  the  eastern 
border  of  the  continent 
was  slightly  warped.  As 
a  result,  the  Triassic  rocks 
are  now  tilted,  to  the  east 
in  New  England,  and  to 
the  west  in  New  Jersey  and  southward.  During  the  same 
disturbance  the  rocks  were  broken  by  many  normal  faults 
which,  in  some  instances,  repeat  the  interbedded  sheets  of 
lava  (Figs.  427  and  428). 

Temporary  inundation  of  the  Northwest.  —  Little  is  known 
of  the  events  of  Jurassic  times 'in  the  great  interior  of  the 

1  It  has  been  suggested  that  some  doubtful  fresh-water  deposits  along 
the  Potomac  River  in  Maryland  are  of  Jurassic  age,  but  it  is  equally  prob- 
able that  they  are  Comanchean  (Lower  Cretaceous). 

405        • 


FIG.  428.  —  The  same,  complicated  by 
several  normal  faults  and  eroded  to  a 
peneplain. 


406 


HISTORICAL  GEOLOGY 


FIG.  429.  — Probable 
appearance  and 
structure  of  a  com- 
mon Jurassic  mol- 
lusk  (Belemnites) 
allied  to  the  mod- 
ern cuttlefish.  The 
shell  is  the  shaded 
portion  below  and 
is  the  only  part 
usually  preserved. 
(After  Pictet.) 


FIG.  431.  — A  Juras- 
sic oyster  shell 
(Ostrea). 


United  States.  In  -the  West  only  barren 
sands  and  clays,  such  as  accumulate  on 
land,  and  probably  under  conditions  of  dry 
climate,  seem  to  have  been  deposited  in 
the  earlier  part  of  the  period.  Later  a 
broad  tract  extending  from  Alaska  south  to 
Wyoming  and  Utah  subsided  enough  to  let 
in  an  arm  of  the  sea.  That  this  sea  was 
shallow  is  indicated  by  the  character  of 
the  sediments,  which  are  shales  and  sand- 
stones, with  occasional  beds  of  limestone. 
The  fossils  in  the  limestones  are  not  only 
the  remains  of  animals  which  lived  in  the 
ocean,  thus  proving  that  this  was  the  water 
of  the  sea  and 
not  of  a  large 
lake,  but  they 
were  most  near- 
ly related  to  the 
animals  which 
lived  at  the 
same  time  on 

the     COast     of  FlG    430.  _  A    large  and    oddly 
Alaska,     and       ornamented   pelecypod    (Trigo- 

even  in  Siberia.'  nia)  of  the  Jurassic  period' 
Their  affinities  with  the  Californian  types 
are  much  less  close.  From  this  we  may 
infer  that  the  sea  came  in  from  the  far 
north  rather  than  from  the  west  (Fig.  432). 
This  inundation  of  the  Northwest  during 
the  late  Jurassic  was  of  short  duration  only. 
At  the  close  of  the  period,  changes  of  level, 

1  At  this  time  Siberia  and  Russia  were  largely  sub- 
merged by  an  expansion  of  the  Arctic  Ocean,  and 
broad  bays  spread  southward  from  this  into  central 
and  western  Europe  joining  the  ancestral  Mediter- 
ranean Sea,  which  was  much  larger  then  than  now. 


THE  JURASSIC  PERIOD 


407 


FIG.  432.  —  Geography  of  North  America  as  it  is  supposed  to  have  been  in 
late  Jurassic  time.     Dotted  pattern  represents  sediments  on  land. 

the  reverse  of  those  which  had  brought  in  the  sea,  again 
excluded  it.  In  neither  case  were  the  strata  disturbed,  and 
where  there  is  any  unconformity  above  the  marine  beds  it 
is  barely  detectable. 

Marine  deposits  along  the  Pacific  slope.  —  On  the  Pacific 
coast  there  was  a  very  different  state  of  affairs.     Throughout 


408  HISTORICAL  GEOLOGY 

the  period  sediments  had  been  accumulating  along  the  margin 
of  the  ocean,  which  at  that  time  spread  eastward  as  far  as 
Nevada.  Probably  deposition  had  been  going  on  along  this 
coast  through  several  of  the  preceding  periods  also.  By  the 
close  of  the  Jurassic  period  the  result  was  a  very  thick  body 
of  marine  sediments,  chiefly  shales  and  sandstones,  which 
had  been  laid  down  in  shallow  water,  and,  as  the  general 
fineness  of  the  material  testifies,  near  a  coast  which  was  not 
rugged. 

Crumpling  on  the  Pacific  border.  —  At  the  end  of  the 
period  this  long  cycle  of  deposition  was  interrupted  by  one  of 
those  intense  crumplings  of  the  earth's  crust,  which  at  inter- 
vals throughout  geologic  history  have  disturbed  first  one 
locality  and  then  another.  By  lateral  compression,  the  driv- 
ing force  of  which  seems  to  have  originated  in  the  deep  basin 
of  the  adjacent  Pacific  Ocean,  the  shales  and  sandstones  were 
closely  folded  so  that  the  beds  now  stand  on  edge.  At  about 
the  same  time,  great  quantities  of  igneous  rock,  especially 
granite,  welled  up  into  the  folded  mass,  and  solidified  in  the 
form  of  stocks  and  huge  batholiths.  On  the  borders  of  these 
intrusions,  the  sedimentary  beds  were  changed  into  schists 
and  other  metamorphic  rocks.  Even  at  a  distance  from  the 
igneous  masses,  the  intense  pressure  exerted  was  sufficient  to 
convert  the  deeply  buried  shales  into  hard  slates,  and,  in 
some  cases,  to  metamorphose  the  rocks  even  more  severely. 
The  result  of  this  series  of  disturbances  was  doubtless  a  wrin- 
kling and  cracking  of  the  surface  of  the  land  parallel  to  the 
Pacific  through  California  and  probably  as  far  north  as 
Alaska.  Mountain  ranges  were  raised  on  the  site  of  what 
had  previously  been  a  shallow  sea.  Even  as  they  were  ele- 
vated, these  mountains  were  being  gradually  dissected  by 
running  water  and  the  other  agencies  of  degradation,  just  as 
the  rising  Sierras  to-day  are  being  sculptured.  In  their  youth, 
we  may  well  suppose  that  these  early  ancestors  of  the  present 
Sierra  and  other  Pacific  ranges  were  lofty  and  rugged  moun- 
tains. They  have  since,  however,  been  worn  down  and  have 


THE  JURASSIC  PERIOD 


409 


totally  disappeared,  the  mountains  which  now  occupy  the 
same  territory  having  been  made  at  a  much  more  recent 
date  by  the  reelevation  of  the  same  strip. 

LIFE  OF  THE  JURASSIC 

Culmination  of  the  reptiles.  —  In  the  Jurassic  period  the 
reptiles,  which  had  been  rising  into  prominence  since  the  Per- 
mian, reached  the  climax  of  their  career.  Type  after  type 
had  made  its  appearance,  until  by  this  time  the  animals  of 
the  reptilian  class  had  assumed  most  of  the  roles  and  taken 


FIG.  433.  — A  Jurassic  dinosaur  (Stegosaurus) ,  as  it  may  have  appeared  in 
life.  (Painted  by  C.  R.  Knight,  under  the  direction  of  Professor  H.  F. 
Osborn.  Copyright  by  Amer.  Mus.  of  Nat.  Hist.) 

possession  of  most  of  the  habitats  which  were  open  to  them. 
The  mammals  and  the  birds  were  then  in  a  very  primitive 
condition,  and  occupied  an  insignificant  place.  The  stations 
which  they  have  since  acquired  were  then  held  by  the  reptiles. 
The  few  rather  small  and  unpretentious  reptiles  which  now 
remain,  —  for  example,  the  snakes,  lizards,  and  turtles,  —  give 
but  a  faint  conception  of  the  saurian  class  in  its  prime.  In  the 
Jurassic,  there  were  also  large  and  ponderous  reptiles  (Fig.  433), 


410 


HISTORICAL  GEOLOGY 


which  more  or  less  resembled  the  great  modern  pachyderms 
such  as  the  elephant  and  the  rhinoceros.  They  fed  upon 
vegetation,  and  in  spite  of  their  bulk  and  the  formidable 
array  of  bony  plates,  scales,  and  spines  with  which  many  of 
them  were  protected,  they  were  probably  neither  ferocious 
nor  dangerous.  There  were  also  smaller  and  more  active 
reptiles  (Fig.  434),  which,  like  the  tigers,  lions,  and  other 
flesh-eating  mammals  of  the  present,  preyed  upon  the  more 
sluggish  varieties  that  fed  on  vegetation. 


FIG.  434.  —  Carnivorous  dinosaurs  (Allosaurus)  of  the  Jurassic  period. 
(Restored  by  C.  R.  Knight,  under  the  direction  of  Professor  Edward  D. 
Cope.) 

Besides  the  land  reptiles,  there  were  batlike  forms  which 
had  developed  the  power  of  flight  almost  as  fully  as  did  some 
of  the  birds  in  later  times.  These  pterosaurs  (Fig.  435),  or 
flying  dragons,  as  they  are  sometimes  called,  had  hollow  bones 
and  other  characteristics  which  are  now  peculiar  to  the  birds. 
One  of  them  had  a  spread  of  wings  of  more  than  twenty  feet, 
—  nearly  twice  that  of  the  largest  living  bird  —  but  the 
majority  were  much  smaller. 


THE  JURASSIC  PERIOD 


411 


FIG.  4oo.  —  The  largest  of  the  pterosaurs.  A  Cretaceous  species.  (Painted 
by  C.  R.  Knight,  under  the  direction  of  Professor  H.  F.  Osborn.  Copy- 
right by  Amer.  Mus.  of  Nat.  Hist.) 

In  the  shallow  waters  along  the  seacoasts  and  in  the 
marshes  of  the  rivers  and  inland  bodies  of  water,  other  reptiles 
which  had  adopted  an  aquatic  mode  of  life  were  abundant. 
Some,  like  the  turtles  and  crocodiles  of  to-day,  divided  their 
time  between  the  water  and  the  shores,  and  were  provided 
with  legs  fairly  well  adapted  for  either  situation ;  while  others, 
as  described  in  the  last  Chapter,  had  become  adapted  for 
swimming  only.  Their  feet  had  been  changed  into  flippers 
not  unlike  those  of  a  whale,  and  in  the  extreme  examples  of 
this  adaptation,  only  the  front  pair  of  paddles  remained. 
The  rear  pair,  being  apparently  less  useful,  had  gradually  dis- 
appeared, as  in  the  modern  whale.  In  such  types  the  loss  of 
the  rear  legs  was  always  compensated  for  by  the  develop- 
ment of  a  long  and  powerful  tail,  flattened  so  as  to  serve  as  an 
efficient  propeller. 

The  mammals  still  in  the  background.  —  The  birds  and 
mammals  have  been  casually  mentioned  as  occupying  a 


412  HISTORICAL  GEOLOGY 

subordinate  place  on  the  stage  of  life  in  the  Jurassic  period. 
The  mammals  had,  indeed,  made  their  appearance  as  early  as 
the  Triassic,  but  they  were  still  very  primitive  and  quite  unlike 
any  forms  which  exist  to-day.  Not  one,  the  remains  of  which 
have  been  discovered,  was  much  larger  than  a  rat,  and  there 
are  reasons  for  believing  that  many  of  them  belonged  to  the 
lowly  group  of  egg-laying  mammals,  which  is  now  extinct 
save  for  the  duckbill  and  spiny  anteater  of  Australia. 


• 


FIG.  436.  —  The    earliest    known    bird    ( Archseopteryx) .     (Modified  after 

Hutchinson.) 

The  earliest  of  the  birds.  —  Our  first  evidence  of  the  exist- 
ence of  the  birds  comes  from  the  Jurassic  rocks  of  Germany. 
In  the  wonderful  lithographic  limestone  of  Bavaria  several 
specimens,  including  even  the  feathers,  have  been  found. 
They  represent  a  bird  (Fig.  436)  which  was  so  unlike  the 
birds  of  to-day  that,  aside  from  the  feathers  and  the  warm 
blood  which  those  feathers  imply,  it  might  with  considerable 
justice  be  looked  upon  as  a  reptile  rather  than  a  bird.  In  its 


THE  JURASSIC  PERIOD  413 

jaws  were  conical  teeth  like  those  of  a  lizard,  and  its  long  tail 
had  bones  out  to  the  very  tip.  The  fingers  of  the  front  limbs 
(or  wings)  were  still  free  and  distinct,  whereas  in  all  modern 
birds  they  have  entirely  grown  together  into  a  single  bone, 
to  which  the  feathers  are  attached.  Its  characteristics,  then, 
are  essentially  intermediate  between  those  of  reptiles  and  of 
birds,  and  seem  to  indicate,  with  more  than  usual  certainty, 
that  the  birds  are  the  direct  descendants  of  some  one  of  the 
earlier  reptiles. 

QUESTIONS 

1.  The  faulted  lava  beds  of  late  Tertiary  age  in  Oregon  and 
Nevada  now  stand  out  as  high  mountain  ranges,   while   those  in 
New  Jersey  make  very  low  mountains,  or  hills.     Can  you  explain  ? 

2.  What  does  the  thin  shale  and  limestone  formation  of  Jurassic 
age  in  northwestern  United  States  tell  us  about  the  Rocky  Moun- 
tains in  that  period  ? 

3.  There  are  many  lava  flows  interbedded  with  the  Jurassic 
shales  in  California,  and  the  shales  contain  marine  fossils.     From 
this  what  do  you  infer  as  to  the  conditions  at  that  time  and  place  ? 

4.  Under  what  conditions  does  molten  lava  form  granite  ? 

5.  Why  are  granites  the  commonest  rocks  in  batholiths  ? 

6.  How  does  it  happen  that  many  batholiths  of  granite  are  ex- 
posed at  the  surface  to-day  ? 

7.  In  what  kind  of  a  deposit  should  the  most  delicate  fossil  be 
most  perfectly  preserved  ? 


B.  &  B.  GEOL.  —  24 


CHAPTER  XXII 
THE   COMANCHEAN   PERIOD1 

Conditions  at  the  beginning  of  the  period.  —  The  opening 
of  the  Comanchean  period  found  the  continent  of  North 
America  very  largely  out  of  water,  the  long  gulf  from  the 
northwest  having  been  excluded  at  the  close  of  the  preceding 
period.  On  the  west  coast  the  series  of  rugged  mountains 
which  had  been  produced  by  the  Sierran  disturbance  were 
being  eroded,  and  the  material  supplied  by  their  decay  was 
spread  along  the  shores  of  the  Pacific  Ocean.  The  present 
Rocky  Mountains  and  most  of  the  numerous  ranges  of  the 
Great  Basin  region  were  not  then  in  existence.  From  the 
Pacific  mountains  to  the  Atlantic  Ocean  there  were  probably 
no  prominent  highlands,  except  some  low  mountains  in  the 
Carolina  region  and  perhaps  others  in  New  England.  By 
this  time  the  folded  ranges  of  the  Appalachian  system  had 
been  worn  down  to  a  lowland  or  peneplain,  over  which  sluggish 
rivers  meandered  on  their  way  to  the  Atlantic  Ocean  and  the 
Gulf  of  Mexico,  and  above  which  a  few  scattered  hills  rose  as 
monadnocks. 

The  beginning  of  the  coastal  plain.  —  Up  to  this  time  the 
coast  line  of  eastern  and  southern  United  States  appears  to 
have  been  considerably  farther  out  toward  the  edge  of  the 
continental  shelf  than  now,  for  almost  no  Paleozoic  or  early 
Mesozoic  rocks  of  marine  origin  have  been  discovered  there. 
In  the  Comanchean,  for  the  first  time,  sediments  were  laid 
down  over  a  considerable  part  of  the  old  Appalachian  land, 
now  represented  by  the  Piedmont  Plateau.  In  the  broad, 
level  flats  and  marshes  back  from  the  coast,  deposits  of  sand 
and  clay  with  occasional  carbonaceous  layers  were  formed. 

1  Often  called  the  Lower  Cretaceous  period. 
414 


THE  COMANCHEAN  PERIOD  415 

These  are  known  locally  as  the  Potomac  series.  Similar 
strata  are  found  in  the  eastern  Gulf  states,  and  there,  too, 
they  are  not  of  marine  origin.  In  Texas  and  Mexico  the  sea 
spread  far  inland  during  the  early  part  of  the  Comanchean 
period.  That  the  submergence  came  on  gradually  and 
disappeared  equally  slowly  may  be  readily  inferred  from  the 
character  of  the  rocks  of  that  age.  Thus  the  lowest  beds  of 
Comanchean  age  in  Texas  do  not  contain  marine  fossils.  They 
are  mixed  sands  and  clays,  with  traces  of  marsh  vegetation, 
—  facts  which  indicate  that  they  were  laid  down  upon  a  low- 
lying  land,  perhaps  in  the  lagoons  and  flats  along  meandering 
rivers.  Upon  these  earlier  strata  shales  and  limestones  are 
found,  and  the  marine  shells  which  they  contain  prove  that 
they  were  deposited  in  the  sea,  which  was  then  advancing 
northward  over  the  land.  Above  the  limestones,  however, 
are  more  sandy  beds,  which  indicate  that  the  shore  line  was 
on  its  retreat  southward,  and  the  sea  was  becoming  shallower, 
until  finally  Texas  and  neighboring  regions  were  left  dry  again. 

Reduction  of  the  Pacific  mountains.  —  On  the  Pacific  slope 
a  vast  thickness  of  gravel,  sand,  and  mud  accumulated  during 
the  Comanchean  period.  No  other  evidence  is  needed  to 
show  that  the  mountains  which  had  been  upraised  at  the  close 
of  the  Jurassic  were  being  rapidly  eroded  and  the  products 
of  their  decay  carried  into  the  adjacent  sea.  Hundreds  and 
doubtless  thousands  of  feet  of  rock  were  carried  away  by  these 
slow  but  incessant  processes,  resulting  in  the  uncovering  of 
even  the  deep-seated  batholiths  of  granite,  which  had  been 
intruded  at  the  time  of  the  folding. 

Emergence  of  the  continent.  —  Above  the  Comanchean 
strata  there  is  almost  everywhere  a  distinct  unconformity, 
which  tells  of  long-continued  erosion  after  the  sediments  were 
deposited.  In  the  Atlantic  and  Gulf  coastal  formations  the 
unconformity  is  merely  an  irregular  surface  dividing  the 
strata  above  from  those  below.  In  that  region  there  was  no 
notable  deformation  between  the  two  epochs  of  deposition. 
On  the  Pacific  coast,  however,  the  unconformity  is  more  promi- 


416  HISTORICAL  GEOLOGY 

nent,  and  in  Mexico  it  is  still  more  conspicuous,  for  there 
fault  scarps  which  were  made  at  the  close  of  the  Comanchean 
were  base-leveled  before  the  Cretaceous  sediments  were  de- 
posited. In  Europe,  also,  an  unconformity  has  been  observed. 
From  the  wide  distribution  of  these  conditions,  it  seems 
probable  that  the  sea  level  was  drawn  down  at  the  close  of 
the  Comanchean  period,  and  that  the  continents  were  again 
largely  out  of  water. 

The  plants  become  modernized.  —  Up  to  this  time  the 
vegetable  world  had  been  represented  entirely  by  such  fossil 
plants  as  the  ferns,  mosses,  and  eye  ads,  —  plants  which  belong 
to  distinctly  lower  groups  than  those  with  which  we  are  now 
most  familiar.  In  size  and  abundance  they  probably  com- 
pared well  with  our  modern  trees,  shrubs,  and  grasses,  but 
they  were  different  in  structure  and  aspect.  In  the  Coman- 
chean period,  for  the  first  time,  the  modern  flowering  plants 
made  their  appearance,  and  in  this  case  with  a  suddenness 
which  is  yet  to  be  explained.  By  the  end  of  the  period  they 
had  become  the  most  abundant  of  all  plants  in  America,  and 
later  in  Europe,  and  they  have  continued  to  hold  their  su- 
premacy ever  since.  It  is  a  significant  fact  that  not  long  after 
the  advent  of  the  flowering  plants  came  the  great  rise  of  the 
mammals,  and  also  the  first  appearance  of  the  higher  insects, 
such  as  the  butterflies,  bees,  and  wasps.  These  animals  are, 
in  fact,  largely  dependent  upon  the  higher  types  of  plant  life 
for  their  existence,  and  their  rapid  development  may  be  due 
in  no  small  measure  to  the  entrance  of  the  Angiosperms 
(p.  294). 

Lesser  changes  among  the  animals.  —  By  a  somewhat 
slower  development,  the  fishes  likewise  had  reached  almost 
their  modern  position  by  the  close  of  the  Comanchean  period. 
The  peculiar  and  in  many  respects  ill-constructed  fishes  of  the 
earlier  periods,  from  the  Silurian  on,  were  gradually  relegated 
to  the  background,  while  the  modern  Teleosts  became  the 
most  abundant  types,  —  fishes  like  our  modern  bass,  salmon, 
and  many  others,  which  have  well-developed  bony  skeletons. 


THE   COMANCHEAN  PERIOD  417 

The  reptiles  had  by  this  time  passed  their  zenith,  but  the 
discussion  of  their  decline  and  the  final  exit  of  most  of  them 
from  the  stage  of  life  is  reserved  for  the  next  Chapter. 

QUESTIONS 

1.  Can  you  suggest  from  Figure  437  how  it  may  be  inferred  that 
the  Appalachian  Mountains  were  worn  down  to  a  lowland  by  the 
Comanchean  period  ? 


FIG.  437.  —  Generalized  section  of  the  Atlantic  slope  of  the  United  States, 
showing  the  relation  of  the  base  of  the  Potomac  sediments  to  the  tilted 
Triassic  beds  and  the  folded  Paleozoic  strata. 

2.  Under  what  conditions  might  a  period  of  emergence  and  land 
conditions   not  be   marked   in  the   section  by  an  unconformity  ? 
Where  in  the  United  States  at  the  present  time  are  these  conditions 
realized  ? 

3.  Can  you  suggest  why  the  Comanchean  system  contains  chalk 
in  Texas  but  marble  in  central  Mexico  ? 

4.  The  Comanchean  rocks  of  California  are  said  to  be  25,000 
feet  thick,  while  in  New  Jersey  they  are  only  700  feet  thick.     What 
reasons  may  be  suggested  for  this  great  difference  ? 

5.  Why  are  bees  now  dependent  on  the  angiosperms  ? 


CHAPTER  XXIII 
THE    CRETACEOUS   PERIOD1 

Renewed  inundation  of  the  continent.  —  Throughout 
the  first  three  periods  of  the  Mesozoic  era  the  seas  had  over- 
spread only  small  portions  of  the  United  States.  The  areas 
submerged  were  (1)  the  Pacific  coast  in  the  Triassic,  with  the 
addition  of  the  northwestern  mountain  region  in  the  Jurassic, 
and  (2)  the  southwest  part  of  the  Great  Plains  in  the 
Comanchean. 

The  Cretaceous  period,  on  the  other  hand,  was  charac- 
terized by  a  widespread  inundation  of  this  and  most  other 
continents.  For  North  America,  at  least,  it  was  the  last 
time  that  any  large  part  of  the  continent  was  covered  by 
the  sea. 

The  coastal  plain  submerged.  —  It  will  be  recalled  that 
along  the  Atlantic  coast  the  Comanchean  strata  are  sands 
and  clays,  which  were  not  deposited  hi  the  sea,  but  more 
probably  along  rivers  and  marshes.  A  series  of  marine  beds 
of  Cretaceous  age  lies  upon  them,  but  the  two  are  separated 
by  a  slight  unconformity.  The  unconformity  represents  an 
interval  during  which  the  plain  was  land,  and  was  subject  to 
erosion. 

The  Cretaceous  beds  are,  like  the  Comanchean,  almost 
unconsolidated,  but  they  differ  in  some  respects  from  the 
latter.  Besides  clay  and  ordinary  sand,  one  of  the  constit- 
uents of  the  Cretaceous,  especially  in  New  Jersey,  is  green- 
sand,  which  seems  to  be  a  chemical  precipitate  formed  in 
relatively  shallow  water. 

1  As  used  here,  the  name  Cretaceous  refers  to  the  "Upper  Cretaceous"  of 
many  writers. 

413 


THE   CRETACEOUS  PERIOD 


419 


Farther  south  in  Alabama  and  Mississippi,  the  Cretaceous 
system  includes  several  hundred  feet  of  chalk  or  soft  lime- 
stone. In  the  belt  where  this  chalky  formation  outcrops,  the 
soil  is  so  good  that  the  cotton  plantations  there  are  the 
richest  in  the  South.  In  the  same  region  the  slave  popula- 
tion was  densest  before  the  Civil  War  and  the  proportion  of 
negroes  is  now  largest. 

An  interior  sea  divides  North  America.  —  Very  early  in 
the  Cretaceous  period  the  sea  which  lay  south  of  the  United 
States  began  a  slow  advance  northward  over  the  nearly 


FIG.  438.  —  Stereogram  of  part  of  the  Great  Plains  early  in  the  Cretaceous 
period,  showing  the  probable  relations  of  the  zones  of  different  kinds  of 
sediments. 

level  surface  of  the  land.  Along  the  shores  of  the  sea  there 
were  probably  marshes  and  lagoons  such  as  now  fringe  the 
low  coast  of  Texas.  Still  farther  inland,  there  were  broad 
plains  over  which  the  sluggish  rivers  were  spreading  fine 
sediments  (Fig.  438). 

As  the  sea  advanced  northward,  the  shore  conditions  must 
have  migrated  slowly  in  front  of  it,  the  streams  constantly 
depositing  sediment  farther  and  farther  north  as  the  advance 
continued.  These  deposits  of  the  alluvial  plains  appear  to 
be  represented  in  the  oldest  Cretaceous  formation  of  the 
Great  Plains  region,  the  Dakota  sandstone.  In  this  sand- 
stone many  leaves  of  land  plants  have  been  found  (Fig.  439). 
As  the  leaves  are  unworn,  and  are  well  preserved,  one  can 
hardly  suppose  that  they  were  washed  out  to  sea  and 


420 


HISTORICAL  GEOLOGY 


there  deposited;  it  is  more 
probable  that  they  fell  di- 
rectly into  the  sandy  shal- 
lows of  rivers  and  bayous, 
and  were  in  turn  covered  by 
the  sands.  Being  a  porous 
layer  between  beds  of  dense 
shale,  the  Dakota  sandstone 
serves  as  an  important  reser- 
voir for  underground  water, 
and  from  it  many  artesian 
wells  in  the  northern  part 
of  the  Great  Plains  derive 
their  flow. 

Beyond  the  shore  line,  in 
the  open  sea,  very  different 
deposits  were  being  laid 
down,  of  course,  —  chiefly 
clays  in  a  broad  belt  near 
the  shore,  and  limy  ooze 
farther  out  in  the  clear  water 
(Fig.  440).  The  shells  and 
bones  of  marine  animals 
became  embedded  in  these 
deposits,  and  are  now  found 
as  fossils  in  the  Cretaceous 
rocks.  These  two  zones,  in 
which  clay  and  ooze  were 
deposited  respectively,  like- 
wise migrated  northward  as 
the  sea  advanced  in  that 
direction,  until  finally  they 
had  overspread  the  Great 
Plains  and  Rocky  Mountain 

FIG.  439.  -  Angiosperm  leaves  from  the          j          f  Oklahoma  and 

Dakota  sandstone.    A,  Poplar  ;  B,  Wil- 
low ;  c,  Sassafras.  Iowa  on  the  east  to  Arizona 


THE  CRETACEOUS  PERIOD  421 

and  Utah  on  the  west,  and  as  far  north  as  the  Arctic 
Ocean.  At  this  time,  then,  North  America  was  divided 
into  two  land  masses  (Fig.  441) :  one  on  the  east,  which, 
being  very  low,  furnished  little  but  fine  sediments  to  the 


FIG.  440.  —  The  same  region  as  in  Figure  438  near  the  middle  of  the 
Cretaceous  period,  showing  the  retreat  of  the  shore  and  the  changed 
positions  of  the  zones  of  sediments. 

interior  sea ;  and  one  on  the  west,  which  was  more  rugged, 
although  by  no  means  so  mountainous  as  the  same  region 
is  to-day. 

By  the  advance  of  the  first  zone  of  marine  deposition 
spoken  of  above,  a  thick  layer  of  clay  was  gradually  built 
up  over  the  Dakota  sandstone,  and  this  in  turn  was  followed 
by  the  zone  of  calcareous  ooze,  which  produced  the  chalk 
now  found  in  Kansas,  Nebraska,  and  the  Dakotas.  The 
chalk  points  to  the  existence  of  a  clear,  open  sea  beyond  the 
reach  of  mud-laden  currents.  In  this  chalk  are  found  not 
only  marine  shells,  but  the  bones  of  large  swimming  reptiles, 
and  even  of  birds  and  flying  reptiles.  In  the  latter  we  seem 
to  have  evidence  that  the  winged  animals  of  Cretaceous 
times  were  accustomed  to  fly  far  out  over  the  sea,  as  gulls 
and  albatrosses  do  now. 

The  interior  sea  retreats.  —  The  duration  of  the  interior 
sea  was  evidently  long,  but  before  the  close  of  the  Cretaceous 
period  changes  began  which  eventually  caused  its  disap- 
pearance. The  sediments  which  were  being  constantly 
swept  into  it  around  its  borders  helped  in  some  degree  to 


422 


HISTORICAL  GEOLOGY 


fill  it  up,  but  the  chief  cause  of  its  withdrawal  is  probably 
to  be  found  in  gentle  changes  of  level, — this  time  the  re- 


FIG.  441.  —  Probable  geography  of  North  America  near  the  middle  of  the 
Cretaceous  period.     The  dotted  pattern  represents  terrestrial  deposits. 

verse  of  those  which  had  caused  it  to  overspread  the  land. 
As  the  sea  receded,  the  zones  of  sedimentation  (ooze,  mud, 
sand,  and  river  deposits)  began  a  slow  retreat.  This  migra- 


THE  CRETACEOUS  PERIOD 


423 


tion  is  recorded  in  the  series  of  clays  and  sands  which  lie 
upon  the  chalk,  and  by  more  sands  containing  coal  seams, 
which  in  turn  are  spread  upon  the 
clays.  The  presence  of  the  coal  seams 
records  the  passing  of  the  shore  line. 
The  retreat  of  the  sea  seems  to  have 
been  somewhat  halting,  however,  and 
interrupted  by  occasional  small  ad- 
vances, for  marine  strata  are  found 
interbedded  with  the  coal-bearing 
sandstones.  That  the  retreat  was 
exceedingly  slow,  and  that  the  land 


FIG.  442.  —  A  pelecypod 
(Inoceramus)  of  the  Cre- 
taceous shales  character- 
istic of  the  western  plains. 


stood  for  a  long  time  not  far  from 
sea  level,  is  suggested  by  the  thick- 
ness of  the  sediments  which  accumu- 
lated and  by  the  number  of  succes- 
sive coal  beds  in  the  upper  part  of 
the  system.  Each  distinct  series  of 
beds  thus  made  has  a  name  of  its 
own,  the  uppermost  or  coal-bearing 
series  being  called  the  Laramie  for- 
mation. There  may  be  as  much  coal 
in  the  Laramie  as  in  the  Pennsylva- 

FIG.    443.  —  A  Cretaceous       .  .  ^ 

gastropod  with  curiously    man  system  in  eastern  United  States, 


formed  shell  and  beaded 
ornamentation. 


Qn 


average   it   is 

of  poorer  quality,  and  but  little  of  it 
is  anthracite. 

Local  deposits  on  the  Pacific  coast. 
—  In  California  and  northward  at 
various  points  as  far  as  Alaska,  Cre- 
taceous sandstones  and  shales  have 
been  recognized.  They  are  usually 
separated  from  the  Comanchean  strata 

/.         • ,       i  i      £     FIG.   444.  —  A   Cretaceous 

by  an  unconformity,  because  much  oi       gea  urchin  or  echinoidt 

the    Coastal    region    Was    land    for    a        with  the  spines  removed. 


424  HISTORICAL  GEOLOGY 

considerable  interval  at  the  close  of  the  Comanchean 
period. 

The  Cretaceous  sediments  were  evidently  derived  from  the 
mountains  which  bordered  the  Pacific  shore.  These  moun- 
tains were  probably  higher  in  the  preceding  period,  but  had 
been  much  reduced  before  Cretaceous  times.  The  thick- 
ness of  the  Cretaceous  sediments  is  correspondingly  less  than 
that  of  the  Comanchean  system.  In  explanation  it  may  be 
suggested  that  the  mountains  had  been  eroded  down  to  a 
subdued  hilly  tract  before  the  Cretaceous  period. 

A  period  of  quiet  throughout  the  world.  —  We  have 
already  seen  how  the  eastern  and  western  portions  of  the 
continent  had  been  reduced  to  lowlands,  either  before  the 
Cretaceous  or  by  the  end  of  that  period.  The  only  notable 
elevations  which  seem  to  have  remained  in  North  America 
were  certain  hills  and  mountains  along  the  Pacific  coast  and 
others  in  the  Carolinas.  The  same  condition  may  be  traced 
in  Europe  and  in  Asia,  where  peneplains  of  enormous  extent 
seem  to  have  developed  by  this  time.  While  the  lands 
were  thus  low  and  monotonous,  comparatively  little  sedi- 
ment was  being  worn  from  them,  and  even  that  was  fine 
mud  and  silt.  In  the  flood  plains  of  the  broad  river  val- 
leys, clays  and  silts  were  spread  in  very  wide  but  thin  layers 
interspersed  with  marsh  deposits ;  while  along  seashores  little 
except  mud  accumulated,  'and  the  deposition  of  pure  lime 
ooze  was  permitted  comparatively  near  the  coast.  Thus 
in  England  and  France,  and  in  many  other  parts  of  the 
eastern  continents,  the  Cretaceous  rocks  consist  largely  of 
chalk  or  limestone. 

Greater  uniformity  of  climate  than  we  now  have  seems 
to  have  been  another  characteristic  of  this  quiet  period  of 
low  lands  and  shallow  seas,  for  plants  much  like  those  of  the 
Gulf  states  lived  also  in  Greenland,  where  snow  and  ice  now 
prevail. 

Conditions  favor  the  sea  animals.  —  The  conditions  of  life 
in  such  a  period  must  have  been  somewhat  more  stable 


THE  CRETACEOUS  PERIOD  425 

than  at  periods  like  the  present,  in  which  comparatively 
rapid  changes  have  been  taking  place.  Uniform  climates 
permitted  the  migration  of  both  animals  and  plants  over 
wide  stretches  of  the  continents  and  of  the  seas.  The 
broadly  expanded  shallow  seas  afforded  a  congenial  field  for 
the  increase  of  marine  life.  Along  with  shells,  corals,  and 
other  remains  of  sea  animals,  we  find  in  the  Cretaceous  rocks 
the  bones  of  many  marine  reptiles,  —  not  only  turtles  like 


FIG.  445.  —  A  Cretaceous  mosasaur.  (Painted  by  C.  R.  Knight,  under  the 
direction  of  Professor  H.  F.  Osborn.  Copyright  by  Amer.  Mus.  of  Nat. 
Hist.) 

those  which  now  inhabit  the  oceans,  but  long,  serpentlike 
forms  in  which  the  legs  were  reduced  to  short  paddles,  whilp 
the  long,  flattened  tail  served  as  a  strong  propeller  (Fig.  445). 
The  sharp  teeth  of  these  mosasaurs  is  ample  evidence  of 
their  ferocious  nature. 

Eccentric  forms  of  the  older  reptiles.  —  The  large  land- 
inhabiting  reptiles,  or  dinosaurs,  which  had  reached  the 
zenith  of  their  career  in  the  late  Jurassic,  were  still  abundant, 
but  had  entered  upon  a  period  of  eccentric  diversification 


426 


HISTORICAL  GEOLOGY 


A  " 


such  as  is  characteristic  of  the  decline  of  many  other  animal 
groups.  Like  their  predecessors,  they  were  ponderous  and 
clumsy  in  the  extreme,  and  the  small 
size  of  the  cavities  in  their  skulls 
shows  how  insignificant  was  the 
capacity  of  their  brains  and  how  little 
intelligence  most  of  them  possessed. 
Externally  they  took  on  many  pecu- 
liar and  apparently  useless  styles  of 
ornamentation,  such  as  the  great 
bony  plates  and  spines  shown  in 
Figure  433. 

The  coiled  shells  called  ammonites 

ornamented     seem  to   ^ave  ^een  m   tne  same  staSe 

with  blunt  spines.    The    of  their  career,  and  likewise  exhibit 

complexity  of  the  sutures     TYlflTlv  nPfM]i;flr 
is  concealed  by  the  shell.  ***  P6< 

forms  and  or- 
naments (Fig.  446).  Some  had  spines, 
others  knobs  or  ridges,  while  some 
showed  a  tendency  to  uncoil  and  re- 
vert to  the  straight  Orthoceras  type, 
although  still  keeping  the  highly 
crumpled  suture  lines  (Fig.  447). 

The  birds  in  transition. — The  birds 
in  the  Cretaceous  period  were  far 
more  like  our  modern  birds  than  was 
the  strange  Archoeopteryx  of  the  Juras- 
sic. In  fact,  the  one  characteristic 
which  linked  them  closely  with  that 
ancestral  form  was  the  possession  of 
teeth  resembling  those  of  reptiles  and 
set  in  sockets  or  grooves  in  the  jaws. 

That  the  birds  had  developed  along  Fio  447.  _  Fragment  of 
several  widely  divergent  lines  is  shown  a  large  partly  uncoiled 
by  the  fact  that  some  which  have  ammonite,  showing  re- 

markable  complexity  of 

been  found  in  the  Cretaceous  rocks       the  suture  lines. 


THE  CRETACEOUS  PERIOD 


427 


were  strong  flyers,  like  the  gulls,  while  others  were  wingless 
(Fig.  449)  and  spent  their  time  exclusively  in  the  water, 
where  they  had  become  as  expert  divers 
and  fishers  as  the  modern  penguins  of 
Antarctica. 

Crustal  disturbances  close  the  period. 
—  Even  before  the  close  of  the  Cretaceous 
period  various  occurrences  gave  a  hint  of 
the  revolutionary  changes  which  finally 
brought  the  period  to  an  end.  In  Mex- 
ico, British  Columbia,  and  elsewhere 
volcanic  eruptions  took  place  on  a  con- 
siderable scale  during  the  later  part  of 
Cretaceous  time.  At  about  the  same  time 
some  portions  of  Colorado,  Wyoming, 
and  doubtless  other  regions  were  warped 
upward.  That  the  resulting  highlands  suffered  rapid  erosion 
is  shown  by  the  coarser  and  more  abundant  sediments  which 
were  deposited  around  their  borders. 

These  disturbances  mark  the  beginning  of  a  widespread 


FIG.  448. —  A  Creta- 
ceous ammonite 
(Turrilites)  with  loose 
spiral  form. 


FIG.  449.  —  A  toothed  diving  bird  (Hesperornis)  of  the  interior  Cretaceous 
sea.  (Painting  by  Gleeson.  By  the  courtesy  of  McClure,  Phillips  and 
Company.) 


428 


HISTORICAL  GEOLOGY 


epoch  of  crustal  deformation,  which  resulted  at  the  close  of 
the  period  in  the  formation  of  many  important  mountain 
ranges,  especially  in  the  western  hemisphere.  Besides  wide- 
spread warping  and  changes  of  level  in  western  United  States, 
the  rocks  were  folded  along  a  belt  from  Mexico  to  Alaska, 
and  also  apparently  the  entire  length  of  South  America. 
This  marks  the  beginning  of  the  present  Rocky  Mountains 


FIG.  450.  —  Gentle  folds  characteristic  of  the  Rocky  Mountains  in  Wyoming. 

and  the  Andes,  although  the  present  height  of  those  moun- 
tains is  due  chiefly  to  later  movements.  The  folding  in  the 
Rockies  at  this  time  was  by  no  means  so  intense  as  it  was 
in  the  Appalachians  at  the  close  of  the  Permian.  The  folds 
are  chiefly  broad  arches  with  troughs  between  (Fig.  450). 
Near  the  Canadian  boundary  the  lateral  compression  was 
relieved  not  only  by  folding,  but  by  a  profound  dislocation, 


FIG.  451.  —  The  great  overthrust  in  the  Rocky  Mountains  of  Montana. 
The  Proterozoic  rocks  on  the  left  have  been  pushed  up  over  the  Mesozoic 
rocks  on  the  right. 

the  older  rocks  having  been  pushed  up  over  the  Mesozoic 
strata  along  a  great  thrust  plane.  At  one  point  the  Algon- 
kian  rocks  have  been  thrust  out  over  the  Cretaceous  beds 
to  a  distance  of  at  least  seven  miles  (Fig.  451). 

While  the  folding  and  warping  were  in  progress,  volcanoes 
came  into  existence  in  many  parts  of  western  America. 
Volcanic  mountains  comparable  to  the  modern  cones  of 
Vesuvius  and  Fujiyama  were  built  up  in  Colorado,  Montana, 


THE  CRETACEOUS  PERIOD 


429 


and  other  western  states,  as  well  as  in  South  America.    These 
were,  however,  only  the  first  of  a  series  of  volcanoes  which 
grew  up  during  the  Tertiary  period  (Fig.  452).     The  earlier 
ones    became    ex- 
tinct  so   long  ago 
that  they  have  been 
worn  down,  but  the 
latest  of  them  we 
still    see    in    such  , 

,  _          FIG.  452.  —  Section  of  a  Tertiary  volcanic  cone  the 

great  peaks  as  IVlt.        upper  part  of  which  has  been  removed  by  erosion. 


The  lava  flows  are  vertically  shaded, 
interbedded  with  fragmental  deposits. 


They  are 


Rainier    and    Mt. 
Shasta. 

Rapid  changes  in  the  animal  life.  —  We  naturally  expect 
to  find  that  the  exclusion  of  the  shallow  seas  which  over- 
lapped the  continent  in  the  Cretaceous  period,  the  growth 
of  mountain  ranges  where  there  had  been  lowlands  before, 
and  the  accompanying  changes  of  climate  had  a  marked 
effect  upon  the  living  things.  Thus  at  the  close  of  the 
Cretaceous  period,  the  ammonites,  which  had  long  been 
abundant  in  the  seas,  died  out  in  a  comparatively  short  time, 
leaving  no  descendants.  Among  the  reptiles  the  change  was 
quite  as  marked,  although  not  as  complete.  Almost  all  the 
great  reptiles  which  were  so  characteristic  of  the  Mesozoic 
era  became  extinct,  and  only  the  smaller  forms  which  we 
have  to-day,  such  as  the  snakes,  lizards,  and  turtles,  lived 
on.  The  crocodiles  seem  to  be  the  only  remaining  represen- 
tatives of  the  large  Mesozoic  reptiles.  ' 

At  the  same  time  the  mammals  began  a  rise  which  in  the 
next  period  became  extraordinarily  rapid.  The  appearance 
of  the  higher  mammals  in  North  America  seems  to  have  been 
sudden,  as  if  they  had  immigrated  from  some  other  locality 
in  which  they  had  slowly  developed  from  simpler  forms.  If 
this  is  true,  the  place  of  their  origin  is  not  yet  known.  Whether 
or  not  the  rapid  spread  and  growth  of  the  mammals  was 
responsible  for  the  disappearance  of  the  great  reptiles  is  an 
open  question,  but  the  suggestion  is  at  least  plausible. 
B.  &  B.  GEOL.  —  25 


430  HISTORICAL  GEOLOGY 

THE  MESOZOIC  ERA  IN  NORTH  AMERICA 

Changes  in  the  form  of  the  continent.  —  At  the  close  of 
the  Paleozoic  era  the  continental  platform  of  North  America 
had  been  left  largely  above  the  sea.  Only  on  the  Pacific 
coast  did  the  ocean  come  farther  inland  than  now.  On  the 
east  side  of  this  extensive  continent  stood  the  rugged  moun- 
tains of  the  Appalachian  system,  perhaps  not  unlike  the 
Andes  of  to-day.  In  the  West  lay  broad,  arid  plains  with 
occasional  salt  lakes,  but  it  is  improbable  that  high  mountains 
stood  there  at  that  time. 

As  the  era  continued,  the  sea  tended  more  and  more  to 
overspread  the  land.  Late  in  the  Jurassic  period  a  long 
gulf  came  in  across  the  depressed  lowland  which  is  now 
occupied  by  the  great  mountains  of  western  Canada.  A 
little  later  the  Atlantic  Ocean  began  to  encroach  upon  the 
eastern  and  southern  border  of  the  continent,  and  along  its 
shores  were  deposited  the  earliest  sediments  of  the  present 
coastal  plain.  Finally,  in  the  Cretaceous  period,  the  depres- 
sion of  the  central  western  part  of  the  continent  allowed  the 
sea  to  submerge  a  broad  strip  extending  from  the  Arctic 
Ocean  to  the  Gulf  of  Mexico,  thus  leaving  North  America 
divided  into  two  smaller  land  masses. 

The  scene  of  earth  movements,  like  that  of  sedimentation, 
was  shifted  to  the  West  in  the  Mesozoic  era.  Not  being 
resurrected  by  further  warping,  the  Appalachian  Mountains 
in  the  East  had  been  gradually  worn  down  to  low  hills  with 
broad  valleys  between.  They  remained  in  this  condition 
through  the  later  part  of  the  era.  The  crumpling  of  the 
Pacific  coast  strip  had  doubtless  produced  a  series  of  great 
mountain  ranges,  among  the  descendants  of  which  are  the 
Sierra  Nevada,  Cascade,  and  Alaskan  ranges  of  to-day.  After 
a  long  period  of  comparative  quiet  during  the  Comanchean 
and  Cretaceous  periods  the  level  strata  of  the  western  in- 
terior were  arched  and  locally  complexly  folded,  thus  estab- 
lishing the  third  great  North  American  mountain  system,  — • 


THE  CRETACEOUS  PERIOD  431 

the  Rocky  Mountains.  By  the  retreat  of  the  inland  sea  at 
about  the  same  time,  the  continent  was  left  more  nearly  in 
its  present  condition  than  ever  before. 

Climatic  conditions.  —  In  the  Mesozoic,  as  in  earlier  eras, 
the  climatic  conditions  left  but  scanty  records  from  which 
we  may  now  draw  inferences.  Extensive  red  beds  give  evi- 
dence of  an  arid  climate  over  large  areas  in  western  United 
States,  Europe,  and  China;  but  such  conditions  may  have 
been  due  to  the  same  local  factors  which  produce  deserts 
to-day.  It  is  thought  that  the  growth  of  abundant  corals 
and  other  tropical  animals  in  northern  Europe  in  the  Jurassic 
period  indicates  a  much  warmer  general  climate  than  the 
present.  There  are  also  differences  in  the  faunas  of  northern, 
middle,  and  southern  Europe  and  North  America  which  may 
be  due  to  climatic  zones.  That  such  zones  have  been  in 
existence  throughout  geologic  history  can  hardly  be  doubted, 
but,  as  already  said,  evidence  of  their  presence  in  the  earlier 
periods  is  scanty.  It  is  quite  probable,  moreover,  that  the 
zones  have  been  more  distinct  at  certain  times  than  at  others. 

Evolution  of  higher  types  of  animals  and  plants.  —  When 
the  Mesozoic  era  began,  the  old  Paleozoic  ferns  and  seed 
ferns  were  sinking  into  a  subordinate  place,  as  the  conifers, 
cycads,  and  other  naked-seed  plants  came  to  the  front. 
Before  the  end  of  the  era,  however,  even  these  were  super- 
seded in  large  measure  by  the  modern  flowering  trees,  shrubs, 
and  grasses.  By  this  change  the  landscapes  doubtless  came 
to  look  much  more  like  those  which  we  now  see. 

Early  in  the  Mesozoic  era  the  reptiles  were  in  the  youth 
of  their  race,  rapidly  developing  and  rising  to  their  zenith 
before  the  Comanchean  period.  Having  mastered  the  life 
of  the  dry  lands,  the  shallow  seas,  the  air,  and  even  the  open 
oceans,  they  kept  their  dominant  place  until  the  end  of  the 
era.  Their  later  years  were  marked  by  inability  to  com- 
pete with  the  rising  mammals,  and  it  is  perhaps  for  this 
reason  that  they  were  soon  relegated  to  the  background. 
Other  groups  of  animals  underwent  corresponding,  if  per- 


432  HISTORICAL  GEOLOGY 

haps  less  striking,  changes ;  and  when  the  next  era  opened, 
all  the  large  groups,  except  the  birds  and  the  mammals,  had 
nearly  reached  their  modern  condition. 

QUESTIONS 

1.  In  the  drier  parts  of  South  Dakota  the  Cretaceous  shales 
contain  many  little  lenses  of  limestone.     These  now  stand  out  as 
conical  hills,  known  as  "tepee  buttes"  from  their  resemblance  to 
an  Indian  tent.     If  the  climate  of  this  region  were  moist  and  the 
surface  densely  forested  these  buttes  would  probably  not  be  formed. 
Why? 

2.  The  Dakota  sandstone  is  exposed  along  the  flanks  of  the 
Rocky  Mountains   in  sharp   ridges,  locally  known  as  "hogbacks." 
What  does  this  tell  about  the  character  of  the  formation? 

3.  Some  of  the  Cretaceous  chalk  is  interbedded  with  layers  of 
sandstone.     What  does  this  indicate  about  the  depth  of  water  in 
which  the  chalk  was  formed  ? 

4.  In  some  of  the  Cretaceous  beds  sticks  of  wood  with  charred 
ends  have  been  found.     What  inference  is  suggested  ? 

5.  Chalk  consists  largely  of  the  shells  of  protozoans.     What  are 
the  habits  of  these  animals  ?     Under  what  conditions  do  they  suc- 
ceed in  forming  a  deposit  of  chalk  ? 

6.  About  the  Black  Hills  of  South  Dakota  the  Laramie  beds 
appear  to  contain  no  material  derived  from  the  Paleozoic  group, 
while  the  Tertiary  beds  which  lie  upon  the  Laramie  are  largely 
composed  of  such  debris.     How  may  this  be  explained  ? 

7.  Bees,  butterflies,  and  many  other  insects  of  like  habits  have 
not  been  found  in  rocks  older  than  Cretaceous.     How  may  this  fact 
be  related  to  evolution  among  the  plants  ? 


CHAPTER  XXIV 
THE    TERTIARY   PERIOD 

Results  of  the  warping  and  folding.  —  The  crustal  dis- 
turbances which  brought  the  Mesozoic  era  to  a  close  wrought 
great  changes  in  the  land  forms  on  the  continent  of  North 
America.  In  the  Appalachian  region  a  broad  swelling  or 
upwarp  of  the  Cretaceous  peneplain1  had  raised  its  surface 
some  two  or  three  thousand  feet,  and  the  streams,  thus  reju- 
venated, were  already  engaged  in  etching  out  the  softer 
strata,  leaving  the  harder  ones  protruding  as  mountain  ridges. 

The  great  central  portion  of  the  country  had  been  raised 
very  little,  but  in  the  Cordilleran  region  of  the  West,  the 
comparatively  low-lying  Mesozoic  surface  had  been  con- 
verted into  mountains  of  considerable  height  with  interven- 
ing basins  and  valleys.  There,  as  in  the  Appalachians,  the 
hills  and  mountains  were  being  worn  down  and  the  resultant 
sediments  were  filling  up  the  lowlands. 

Divisions  of  the  period.  —  The  Tertiary  period,  while  per- 
haps no  longer  than  many  that  preceded  it,  is  of  course  much 
better  known,  because  it  is  nearer  the  present.  It  is  usually 
divided  into  several  epochs  2 :  — 

(3)   Pliocene  (more  recent). 

(2)    Miocene  (less  recent). 

(1)    Eocene  (dawn  of  the  recent). 

Additions  to  the  Atlantic  and  Gulf  coastal  plain.  —  From 
New  England  south  to  Florida,  and  almost  encircling  the 
Gulf  of  Mexico,  the  Tertiary  sediments  are  found  lying  upon 
the  Comanchean  and  Cretaceous  deposits  which  had  formed 

1  See  page  414. 

2  Of  these,  the  Eocene  is  probably  much  longer  than  either  of  the  others, 
and  is  often  divided  into  Eocene  (proper)  and  Oligocene. 

433 


434 


HISTORICAL  GEOLOGY 


FIG.  453.  —  Supposed  outline  of  North  America  early  in  the  Tertiary 
period.  The  dotted  pattern  represents  deposits  made  on  land.  (Modi- 
fied after  Willis.) 

the  beginning  of  the  coastal  plain.  Some  of  these  Tertiary 
rocks  are  sand,  peat,  clay,  and  marl,  and  some  are  soft  lime- 
stone or  chalk.  They  are  interrupted  by  a  number  of  slight 
unconformities.  In  these  deposits  and  unconformities  we 
find  recorded  the  fact  that  the  eastern  and  southern  margin 


THE  TERTIARY  PERIOD  435 

of  the  continent  was  sometimes  submerged,  and  was  thus 
the  site  of  deposition ;  and  that  at  other  times  it  was  out  of 
water,  and  was  liable  to  erosion.  The  changes  of  level, 
whether  of  the  land  or  of  the  sea,  were  not  great  in  any  in- 
stance. One  result  of  the  slight  warping  to  which  eastern 
United  States  was  subjected  during  the  Tertiary  period  was 
the  formation  of  an  island  within  the  present  confines  of 
Florida,  and  later  the  addition  of  this  island  to  the  mainland 
in  the  form  of  a  peninsula.  No  deposits  older  than  the  Ter- 
tiary limestones  are  exposed  in  that  state. 

At  several  places  in  Texas,  Louisiana,  and  California  wells 
drilled  down  into  the  Tertiary  sediments  have  yielded  petroleum. 
This  oil,  and  the  natural  gas  which  usually  accompanies  it,  is  prob- 
ably produced  by  the  slow  decomposition  of  animal  and  vegetable 
matter  which  was  mixed  with  the  sediments  at  the  time  they  were 
deposited.  Certain  sandy  beds  become  saturated  with  the  gas  and 
liquid  and  when  one  of  these  is  pierced  by  the  drill  a  flowing  well 
may  result.  In  some  cases  the  gas  pressure  is  so  great  that  the  oil 
is  blown  out  in  a  jet.  These  "gushers"  often  wreck  the  buildings 
and  derricks  over  the  wells,  and  much  oil  is  wasted  before  the 
fountain  can  be  controlled. 

Only  a  part  of  the  oil  produced  in  the  United  States  comes  from 
Tertiary  beds.  That  of  Ohio  and  Indiana  is  in  the  Paleozoic  rocks, 
and  the  Kansas  oil  is  only  a  little  younger.  Doubtless  the  condi- 
tions for  the  formation  of  gas  and  oil  have  been  present  somewhere 
in  all-  the  geologic  periods. 

Local  sedimentation  on  the  Pacific  coast.  —  In  the  Ter- 
tiary period,  the  Pacific  coast  was  apparently  somewhat 
abrupt  and  rugged,  although  perhaps  less  so  than  it  is  to-day. 
Erosion  was  the  chief  activity  along  the  western  slope. 
Here  and  there,  however,  deposits  of  Tertiary  age  have  been 
found,  those  in  the  coast  ranges  of  California  being  largely  of 
marine  origin,  while  farther  north,  near  Puget  Sound  and 
in  Alaska,  early  Tertiary  beds  containing  coal  seams  are 
known.  The  latter  were  evidently  laid  down  in  swampy 
lowlands  near  the  coast,  but  not  submerged  by  the  sea. 

Alluvial  deposits  in  the  Great  Plains.  —  Throughout  most 
of  the  Tertiary  period  the  Great  Plains  were  much  nearer 


436  HISTORICAL  GEOLOGY 

sea  level  than  now,  and  less  intrenched  by  valleys.  Many 
streams  which  issued  from  the  newly  made  mountains  on 
the  west  were  spreading  their  loads  of  gravel,  sand,  and  mud 
far  and  wide  over  the  low-lying  surface.  Here  and  there 
lakes  and  marshes  doubtless  existed  temporarily,  and  the 
location  of  these  shifted  from  time  to  time,  so  that  the  de- 
posits which  now  represent  the  Tertiary  in  the  Great  Plains 
are  partly  such  as  are  laid  down  in  lakes,  and  partly  those 
which  rivers  and  even  winds  make.  In  the  Tertiary  epochs 
the  climate  of  the  Great  Plains  region  was  on  the  average 
moister  than  it  is  to-day.  Coaly  layers  in  the  Tertiary 
strata  indicate  the  existence  of  swamps,  where  now  only 
dry  prairies  are  to  be  found.  The  Tertiary  deposits,  which 
have  since  been  elevated  and  subjected  to  a  drier  climate, 
are  now  being  rapidly  dissected  by  the  growth  of  ramifying 
valleys  and  tributary  gullies.  In  parts  of  Dakota  and 
Montana  the  result  is  an  extremely  rugged  complex  of 
ridges,  mesas,  and  buttes,  over  which  travel  is  very  difficult, 
and  which  are  therefore  known  as  "  Bad  Lands  "  (Fig.  454). 

Changes  affecting  the  western  mountains.  —  The  young 
Rocky  Mountains  and  others  farther  west  were  being  rapidly 
worn  down  by  the  activities  of  wind,  rain,  and  streams. 
Some  of  the  material  thus  furnished  found  lodgment  in  the 
interior  basins  between  the  mountain  ranges,  and  there 
accumulated  to  great  thickness.  As  in  the  plains,  these 
deposits  were  made  partly  in  lakes,  but  are  to  be  ascribed 
in  large  measure  to  the  work  of  streams  which  built  alluvial 
fans  in  front  of  the  valleys  they  had  cut  in  the  mountain 
slopes.  Coalescing  with  each  other,  these  fans  came  to  form 
alluvial  plains. 

In  addition  to  the  sand,  gravel,  and  silt,  beds  of  volcanic 
ash  and  sometimes  of  coarser  tuff  are  found  included  in  these 
Tertiary  strata.  They  record  the  eruptions  which  took 
place  at  intervals  from  the  volcanoes  in  Colorado,  Montana, 
and  many  other  western  states  while  the  Tertiary  sediments 
were  being  laid  down.  The  old  volcanic  cones  have  been 


THE   TERTIARY  PERIOD 


437 


-'« 


FIG.  454.  —  Bad  Land  topography  in  South  Dakota.     (Darton,  U.S.  Geol. 

Sun.) 

slowly  worn  down,  but  their  cores  and  remnants  of  the  old 
lava  flows  may  still  be  recognized. 

Most  of  the  ore  deposits  which  have  given  the  western  states 
their  renown  as  mining  districts  are  connected  with  the  volcanic 
intrusives  of  Tertiary  times.  The  gold,  the  discovery  of  which 
caused  the  rush  of  immigrants  to  California  in  1849  and  succeeding 
years,  was  found  partly  in  gravels  in  the  valleys  of  Tertiary  rivers. 
The  famous  gold  mines  of  Cripple  Creek,  Colorado,  and  the  copper 
mines  of  Butte,  Montana,  and  parts  of  Utah,  all  get  their  ores 
from  veins  adjacent  to  bodies  of  porphyry  and  other  igneous  rocks 
which  were  forced  into  the  older  formations  in  the  Tertiary  period. 
In  this  respect  there  is  a  contrast  between  the  western  and  eastern 
mountains  of  the  United  States. 

The  climate  of  the  mountain  region  could  not  have  been 
as  arid  as  it  'is  to-day,  for  the  luxuriant  vegetation  which 
flourished  there  in  the  Tertiary  period  shows  that  the  rain- 
fall was  plentiful  In  some  of  the  driest  parts  of  Utah  and 


438 


HISTORICAL  GEOLOGY 


Wyoming  the  Tertiary  strata  have  preserved  abundant  leaves  of 
palms,  figs,  and  magnolias.  The  present  dryness  is  doubtless  to 
be  ascribed  in  part  to  the  later  uplifting  of  the  present  mountain 
ranges  which  shut  off  the  moist  winds  from  the  Pacific  Ocean. 
Mountain  growth.  —  In  the  West,  the  Eocene  epoch  was 
occupied  largely  in  the  wearing  down  of  the  highlands  which 
had  been  produced  at  the  close  of  the  Cretaceous  period, 
and  in  the  filling  of  the  lowlands.  Warping  and  volcanic 
activity,  although  they  had  not  ceased,  were  of  minor  im- 
portance. It  was  an  epoch  of  quiescence. 


FIG.  455.  —  Trend  lines  of  folds  made  during  the  middle  Tertiary  epoch  of 
mountain-building. 

Near  the  middle  of  the  Tertiary  (Miocene  epoch),  how- 
ever, the  disturbances  were  renewed  on  quite  as  grand  a 
scale  as  before,  but  in  part  along  different  lines.  One  of  the 
greatest  results  of  this  deformation  is  now  seen  in  the  series 
of  high  mountain  chains  which  partially  encircles  the  globe 
north  of  the  equator  (Fig.  455).  In  our  own  hemisphere  it 


THE   TERTIARY  PERIOD 


439 


is  represented  in  the  mountains  of  Cuba,  Porto  Rico,  and 
southern  Mexico  (Antillean  system).  (See  Fig.  44.)  These 
ranges  are  less  conspicuous  than  some  of  the  mountains  on 
the  land,  only  because  they  are  largely  submerged.  The 
highest  peaks  of  Cuba  rise  more  than  twenty-five  thousand 
feet  above  the  floor  of  the  adjacent  Caribbean  Sea.  In  the 
old  world  the  eastward  trending  mountains,  from  the  Pyre- 


FIG.  456.  —  Geography  of  the  world  as  it  is  thought  to  have  been  at  a  time 
early  in  the  Tertiary  period.  Note  the  continuous  land  in  the  northern 
hemisphere,  with  isolated  continents  in  the  south. 

nees  in  Spain,  through  the  Alps,  Caucasus,  and  many  other 
ranges,  to  the  Himalayas  and  far  beyond,  belong  to  this 
great  belt  of  Tertiary  mountains.  Hitherto  most  of  these 
regions  had  been  beneath  the  sea;  on  the  site  of  even  the 
great  Himalayas  there  was,  up  to  the  early  part  of  the  Ter- 
tiary period,  a  broad  sea,  not  unlike  the  Mediterranean 
(Fig.  456).  In  this  sea  limestone  was  being  quietly  formed. 
But  in  the  Tertiary  disturbance  these  and  all  older  rocks 


440  HISTORICAL  GEOLOGY 

of  this  locality  were  folded,  compressed,  and  raised  into  lofty 
ridges  which  are  now  being  carved  by  erosion  into  rugged 
mountains. 

A  little  later,  the  Sierras,  Rockies,  and  other  western 
ranges  began  a  renewed  epoch  of  growth;  this  time  not 
chiefly  through  folding,  as  at  the  close  of  the  Cretaceous 
period,  but  by  mere  warping  and  faulting.  The  rise  of  the 
Sierra  range  and  its  northward  continuations  consisted  of  an 
arching  of  the  surface;  but  locally,  as  along  the  east  base 
of  the  Sierra,  the  arch  cracked  (Fig.  457),  or,  in  other  words, 
was  faulted,  and  that  side  is  now  much  steeper  than  the 
slope  toward  the  Pacific.  There  is  good  evidence  that  the 
slow  uplifting  of  the  Sierra  is  still  going  on,  for  as  recently 
as  1872  a  slip  of  nearly  twenty-five  feet  occurred  along  this 
fault  plane. 


FIG.  457.  —  Stereogram  of  a  low  fold  broken  on  one  side. 

Tertiary  volcanoes.  —  In  this  western  region  volcanic 
eruptions  continued,  but  with  somewhat  decreasing  activity. 
They  have  only  very  recently  ceased,  and  it  is  in  fact  by 
no  means  certain  that  the  present  is  anything  more  than 
a  temporary  period  of  quiet  in  that  respect.  Near  the 
middle  of  the  Tertiary  period,  eruptions  of  lava  from  fissures 
as  well  as  from  volcanic  craters  took  place  over  a  vast  area 
in  the  northwestern  part  of  the  United  States,  particularly  in 
Idaho,  Washington,  and  Oregon.  Flow  after  flow  of  liquid 
lava  welled  up  through  cracks  in  the  earth  and  poured  out 
over  the  surface,  leveled  up  its  inequalities,  and  finally  pro- 
duced a  plateau  more  than  a  thousand  feet  in  height  and  equal 
in  extent  to  several  good-sized  states  (Figs.  25,  458).  Simi- 
lar eruptions  have  occurred  occasionally  in  earlier  periods, 
but  nothing  quite  like  them  has  been  observed  in  historic  times. 


THE   TERTIARY   PERIOD  441 

The  renewal  of  the  uplifts  late  in  the  Tertiary  and  con- 
tinuing into  the  next  period  brought  on  the  conditions 
which  we  now  think  of  as  characteristic  of  the  region.  It  is 
to  these  later  movements  that  the  present  elevation  of  our 


FIG.  458.  —  Stereogram   of  a  part  of  the   Columbia  River  lava  plateau, 
showing  flows  with  interbedded  layers  of  sand  and  gravel. 

high  ranges  is  due ;  and,  as  has  been  said,  in  some  of  them 
the  growth  is  still  in  progress.  During  the  uplifts,  the 
streams  sank  their  valleys  deeper  and  deeper  into  the  lands, 
so  that  the  West  is  now  characterized,  not  only  by  moun- 
tain ranges,  but  by  high  plateaus  deeply  cut  by  canons, 


FIG.  459.  —  Young  fault  block  mountains  in  southern  Oregon.     (Modified 

after  Davis.) 
Why  are  the  depressed  spaces  between  the  blocks  flat? 

such  as  those  of  the  Colorado  and  the  Snake  rivers.  The 
growth  of  the  mountains  also  deprived  the  winds  from  the 
Pacific  of  a  large  part  of  their  moisture,  and  thus  condemned 
the  interior  basins  and  the  Great  Plains  to  a  much  drier 
climate  than  they  had  before. 


442  HISTORICAL  GEOLOGY 

LIFE  or  THE  TERTIARY  PERIOD 

Modern  aspect  of  the  lower  forms  of  life.  —  Before  the 
Tertiary  period,  all  the  important  types  of  plants  had  made 
their  appearance,  and  the  flowering  group  had  taken  the 
place  it  now  holds  in  the  lead.  The  lower  groups  of  animals 
had  likewise  become  much  like  those  we  have  to-day.  The 
trilobites,  brachiopods,  ammonites,  and  other  ancient  divi- 
sions had  given  way  to  modern  groups  of  crustaceans,  bi- 
valves, cuttlefish,  and  others.  The  fishes,  amphibians,  and 
reptiles  had  passed  their  prime  and  were  represented  in  the 
Tertiary  period  only  by  species  resembling  those  now 
living. 

Only  the  birds  and  mammals,  then,  claim  our  interest, 
because  they  alone  are  still  progressing.  Of  these,  the 
mammals  are  much  the  more  important,  and  have  left  us  the 
better-preserved  fossils.  They  are  now  the  highest  and  most 
powerful  of  the  animal  groups. 

Generalized  mammals  of  the  Eocene  epoch.  —  Among 
the  beds  of  sand  and  clay  which  were  laid  down  in  the  broad 
Eocene  valleys  of  our  western  mountain  region  and  certain 
other  parts  of  the  world,  abundant  skeletons  of  mammals 
have  been  found.  They  show  that  many  kinds  were  even 
then  in  existence,  that  they  differed  considerably  in  their 
habits  of  life,  and  that  they  were  already  the  leading  ani- 
mals of  their  time.  At  the  present  day  we  have  no  difficulty 
in  distinguishing  the  several  large  groups  of  mammals  from 
each  other.  Thus  we  have  the  flesh  eaters  (Carnivores), 
such  as  the  tiger,  bear,  and  wolf ;  the  hoofed  mammals  (Un- 
gulates), such  as  the  horse,  buffalo,  and  deer;  the  gnawers 
(Rodents),  such  as  the  squirrel  and  rat;  the  whales  and 
dolphins  (Cetaceans),  which  are  swimmers  exclusively;  and 
still  others.  It  is  difficult,  however,  to  place  the  early  Ter- 
tiary mammals  in  these  familiar  divisions.  Instead,  we  find 
varieties  which  seem  to  have  combined  the  characteristics  of 
several  later  groups.  For  example,  it  is  possible  to  trace 


THE  TERTIARY  PERIOD 


443 


the  horse,  deer,  and  rhinoceros  families,  with  their  specialized 
hoofs  and  grinding  teeth,  back  to  a  peculiar  five-toed  animal 
which  had  a  full  set  of  rather  simple  teeth,  and  was  no  larger 
than  a  dog  (Fig.  460).  This  Eocene  form  seems  to  be  an 
ancestral  or  generalized  type  from  which  the  later  hoofed  ani- 
mals diverged  and  ascended.  Furthermore,  it  resembles  in 
many  respects  the  equally  generalized  ancestors  of  the  dogs, 
bears,  and  cats,  although  cats  and  horses,  for  example,  to-day 
seem  to  have  little  in  common. 


FIG.  460.  —  A  generalized  hoofed  mammal  (Phenacodus)  which  lived  in 
North  America  near  the  beginning  of  the  Tertiary  period.  (Painted  by 
C.  R.  Knight,  under  the  direction  of  Professor  H.  F.  Osborn.  Copyright 
by  the  Amer.  Mus.  of  Nat.  Hist.) 

Rapid  evolution  of  the  mammals.  —  The  progress  of 
these  generalized  mammals  of  the  early  Eocene  was  aston- 
ishingly rapid.  In  each  later  series  of  deposits  the  bones  of 
new  and  more  modern  varieties  are  found.  Thus,  before  the 
middle  of  the  Tertiary  period  (Miocene),  the  main  divisions 
of  the  mammals  became  entirely  distinct  and  we  may  easily 
recognize  cats,  horses,  monkeys,  whales,  bats,  elephants,  and 


444 


HISTORIC -L  GEOLOGY 


many  other  kinds.  True,  they  were  not  the  same  species 
which  exist  to-day ;  some  of  the  horses,  for  example,  had  three 
toes  instead  of  one,  as  they  now  have ;  but  the  types  were  un- 
mistakable. Before  the  close  of  the  Tertiary,  the  older  and 
more  primitive  mammals  had  been  exterminated  from  the 
northern  continents,  and  the  whole  animal  kingdom  had 
taken  on  very  largely  its  present  aspect. 


FIG.  461.  —  Ancestral  Eocene  horses  (Eohippus)  with  three  and  four  toes 
on  the  feet.  (Painted  by  C.  R.  Knight,  under  the  direction  of  Professor 
H.  F.  Osborn.  Copyright  by  Amer.  Mus.  of  Nat.  Hist.) 

The  mammals  adopt  many  modes  of  life.  —  As  mentioned 
in  Chapters  on  the  Mesozoic  era,  the  reptiles  when  in  their 
prime  had  occupied  the  forests,  the  plains,  the  marshes,  the  seas, 
and  all  other  situations  in  which  animals  could  well  exist.  In 
the  Tertiary  period  we  find  the  mammals  stepping  into  the 
places  relinquished  by  the  reptiles,  perhaps  after  having  actu- 
ally displaced  them  by  sheer  victory  in  competition.  Thus 


THE  TERTIARY  PERIOD  445 

we  have  mammals  of  the  forest  (for  example,  squirrels),  of  the 
plains  (antelopes),  of  the  marshes  (beavers),  of  the  air  (bats), 
of  the  ocean  (whales),  and  many  more. 

Interestingly  enough,  as  the  mammals  adopted  these  modes 
of  life,  they  often  took  on  in  a  degree  the  form  and  appearance 
of  their  reptilian  predecessors.  To  appreciate  this  one  has 
only  to  compare  the  bat  with  the  pterosaur  (Fig.  435),  the 
porpoise  with  the  fish  reptiles  (Fig.  421),  and  the  rhinoceros 
with  the  heavy  dinosaurs  (Fig.  433). 

Migrations  of  the  Tertiary  mammals.  —  There  are  to-day 
some  very  peculiar  things  about  the  distribution  of  certain 
animals  which  are  explained  only  when  we  study  the  fossils 
from  the  Tertiary  formations.  The  camels  are  now  found 
in  Asia  and  Africa,  and  also  in  the  Andes  Mountains  of  South 
America.  In  Tertiary  times,  as  the  fossils  show  us,  they 
roamed  widely  over  western  North  America  as  well,  and  it 
seems  probable  that  they  migrated  thence  to  Eurasia  by  way 
of  Alaska  at  a  time  when  that  peninsula  was  less  submerged 
than  now  and  enjoyed  a  warmer  climate.  Later  they  died 
out  in  North  America.  This  is  but  an  instance  of  many 
migrations  by  which  the  mammals  of  Eurasia  and  America 
mingled  during  the  Tertiary  period. 

Some  islands  were  so  isolated  by  water  that  they  could 
not  be  reached  by  the  mammals  which  originated  in 
the  larger  continents.  Australia  is  a  case  in  point. 
There  we  find  almost  none  of  our  familiar  higher  mammals, 
but  instead  a  host  of  peculiar  marsupials,  among  which 
are  kangaroos,  wombats,  and  opossums.  It  is  known  that 
these  marsupials  are  most  closely  related  to  animals  that 
lived  in  Europe  in  the  Mesozoic  era,  but  died  out  there  earty 
in  the  Tertiary  period.  The  inference  is  that  Australia  has 
been  isolated  from  the  other  lands  since  perhaps  the  Creta- 
ceous period,  and  that  during  the  Tertiary  period  her  peculiar 
mammals  evolved  along  their  own  lines  without  that  inter- 
ference which  comes  from  sharp  competition  with  the  more 
progressive  higher  animals. 

B.  &  B.  GEOL.  - — 26 


446  HISTORICAL  GEOLOGY 

South  America  and  Africa  were  similarly  isolated  at  certain 
times,  but  later  in  the  Tertiary  period  they  were  linked  with 
the  northern  lands  and  thence  received  the  tide  of  immigrants 
belonging  to  the  more  advanced  mammals. 

By  studying  the  present  distribution  of  animals  and  work- 
ing out  the  paths  of  their  earlier  migrations,  we  can  learn 
much  about  the  changes  which  have  taken  place  in  geography 
during  the  later  periods.  The  map  (Fig.  456)  shows  roughly 
how  the  continents  and  seas  are  thought  to  have  been  ar- 
ranged in  early  Tertiary  times,  as  compared  with  the  present. 

QUESTIONS 

1.  The  Eocene  coal  in  the  vicinity  of  Seattle  is  bituminous  and 
locally  even  anthracitic  ;    that  in  Mississippi  is  soft  lignite.     With- 
out further  information,  what  predictions  would  you  venture  as  to 
the  geological  conditions  in  the  two  regions  ? 

2.  It  has  been  suggested  that  the  climatic  changes  known  to 
have  taken  place  in  the  Tertiary  period  may  have  been  caused 
partly  by  changes  in  the  ocean  currents.      What  would    happen 
to-day  if  Florida  joined  Cuba  and  the  Bahama  Islands,  while  at 
the  same  time  Central  America  were  submerged  deeply  ? 

3.  Can  you  suggest  why  the  known  deposits  of  Eocene  age  are 
largely  those  which  were  made  on  the  surface  of  the  land  ? 

4-  Tne  accompanying  diagram  (Fig. 
462)  represents  a  mountain  range  in  the 
West.  Show  how  the  depth  of  erosion 
early  in  the  Tertiary  may  be  estimated 

FIG.  462. -Section  of  moun-  from  Such  a  Section' 
tain    range.     The    folded          5.    The   Eocene   strata  of  Wyoming 

beds    are    Paleozoic     and  include    beds    of    limestone   with    fossil 

Mesozoic.     The  horizontal  fishes.    From  your  knowledge  of  Tertiary 

^°^^  what  do  you  SUSPect  Was  *he 
origin  of  these  beds  ? 

6.  What  events  or  conditions  are  recorded  in  the  following  sec- 
tions (Figs.  463,  464,  and  465)  selected  from  the  Tertiary  strata  in 
different  parts  of  the  United  States? 

7.  The    pyramids   of    Egypt   were    built 
of  Eocene  limestone.    What    change   must 


have  taken  place  in  this  deposit  of  shells    pIG   453  Limestone 

since  it  was  formed  ?  (Florida).    >    •'; 


THE  TERTIARY  PERIOD 


447 


8.   In  Thibet,  marine  limestone  of  Eocene  age  has  been  found  at 
altitudes  of  20,000  feet.     How  much  of  the  history  of  Thibet  may 
be  inferred  from  this 
fact? 

9.  On  the  west- 
ern slope  of  the 
Sierra  Nevada  range 
there  are  flat-topped 
ridges  capped  with 
sheets  of  lava  (Fig. 
466).  Beneath  the 
lava  gold-bearing 
gravels  have  been 
found.  Can  you 
suggest  how  the 
present  conditions 


FIG.  465.  —  Conglomerate, 
sandstone,  and  shale,  in 
contact  with  a  mass  of 
granite  (Oregon). 


FIG.  464.  —  Alternate 
shale,  sandstone,  and 


conglomerate  (Colo-   were  brought  about  ? 


rado). 


What  would  be  the 


FIG.  466.  —  Diagram  of  a 
lava-capped  ridge  in  Cali- 
fornia. 


best  method  of  mining  the  gold  in  these  deposits  ? 


CHAPTER  XXV 
THE    QUATERNARY   PERIOD 

The  great  ice  sheets.  —  During  the  later  part  of  the  Ter- 
tiary period  the  climate  of  the  northern  regions  was  becom- 
ing somewhat  colder,  so  that  palms  no  longer  flourished  in 
Greenland,  nor  corals  off  the  coast  of  Scotland,  as  they  had  in 
the  early  Tertiary.  In  the  Quaternary  period,  from  causes  not 
yet  understood,  the  temperature  of  the  northern  regions  had 
been  lowered  to  such  a  degree  that  the  snows  of  winter  were 
not  melted  off  in  summer.  Thus  glaciers  came  into  existence, 
not  only  in  high  mountains  and  polar  regions  where  we  have 
them  to-day,  but  over  large  regions  which  are  now  free  from 
them.  Through  the  long  accumulation  of  snows,  thick  ice 
sheets,  or  continental  glaciers,  grew  up  in  North  America  and 
in  Scandinavia  and  spread  outward  in  all  directions  until 
they  covered  Canada  and  much  of  Europe. 

In  North  America  the  ice  sheets  extended  into  the  United 
States  as  far  south  as  southern  Illinois  and  New  Jersey. 
Singularly  enough,  they  did  not  cover  much  of  Alaska,  in 
spite  of  the  fact  that  it  is  farther  north  than  some  of  the  coun- 
tries which  were  glaciated  (Fig.  467). 

The  fact  that  ice  sheets  did  not  cover  Alaska  and  Siberia, 
two  of  the  coldest  parts  of  the  world,  shows  that  low  tempera- 
ture was  not  the  only  condition  needed  to  bring  on  glaciation. 
Plenty  of  snow  is  likewise  essential,  and  so  in  rather  dry 
regions  or  where  there  are  short,  hot  summers,  even  where 
there  is  great  cold,  we  find  no  glaciers. 

Successive  advances  and  retreats  of  the  ice.  —  The  Glacial 
epoch  was  marked  by  the  growth  and  eventual  melting  off  of 
not  merely  one  ice  sheet  but  of  several,  one  after  the  other. 
This  is  true  of  both  Europe  and  North  America.  In  the 

448 


(449) 


450  HISTORICAL  GEOLOGY 

Mississippi  Basin  evidence  of  several  advances  and  retreats 
has  been  discovered.  The  greatest  extension  was  reached 
by  the  second  ice  sheet,  which  spread  southward  almost  to 
the  mouth  of  the  Ohio  River ;  but  later  ones  fell  only  a  little 
short  of  that.  Between  the  several  advances,  the  ice  sheets 
seem  to  have  entirely  disappeared  or  to  have  been  reduced  to 
much  smaller  size.  That  these  disappearances  were  caused 
by  periods  of  warmer  climate  is  shown  by  the  finding  of  leaves 
of  southern  plants  in  clay  beds  between  two  layers  of  glacial 
till  as  far  north  as  Toronto  in  Canada.  Trees  now  charac- 
teristic of  the  Ohio  Valley  then  lived  abundantly  north  of 
Lake  Erie. 

In  view  of  the  fact  that  ice  sheets  grind  down  the  surface 
over  which  they  slowly  creep,  we  need  not  wonder  that  the 
later  ice  sheets  removed  much  of  the  deposits  left  during  pre- 
ceding glaciations.  Even  where  the  earlier  sheets  of  drift  were 
not  destroyed,  they  are  now  largely  buried  by  deposits  of  the 
later  ice  sheets.  We  therefore  know  the  older  drift  best  around 
the  edges  of  the  newer.  Its  greater  age  is  indicated  clearly 
by  the  fact  that  it  has  been  deeply  trenched  by  branching 
systems  of  valleys  which  have  been  growing  and  extending 
themselves  through  all  the  time  since  the  early  deposits  of 
drift  were  laid  down.  The  last  drift  sheet  was  made  so  re- 
cently that  the  streams  have  barely  begun  this  work  of  trench- 
ing, and  its  usually  rough  surface  is  still  dotted  with  undrained 
lakes  and  marshes. 

Estimates  of  the  length  of  the  Quaternary  period.  —  Many 
attempts  have  been  made  to  estimate  the  number  of  years 
represented  by  the  glacial  advances  and  retreats.  At  present 
the  cliff  at  Niagara  Falls  is  being  cut  back  several  feet  per 
year.  It  has  been  calculated  that  at  some  such  rate  it 
would  take  from  7,000  to  50,000  years  to  cut  the  entire  gorge 
below  the  falls.  Since  the  falls  could  not  have  begun  until 
after  the  last  ice  sheet  had  retreated  to  Lake  Ontario,  a  some- 
what longer  time  would  be  required  to  take  us  back  to  the 
beginning  of  the  retreat  of  the  latest  glaciers.  By  compar- 


THE  QUATERNARY  PERIOD  451 

ing  the  effects  of  weathering  and  erosion  on  the  older  and 
younger  sheets  of  drift  it  is  possible  to  gain  a  rough  idea  as  to 
their  relative  ages.  Estimates  thus  made  of  the  length  of 
time  since  glaciation  began  range  from  500,000  to  1,500,000 
years.  It  is  impossible  to  make  a  much  closer  calculation 
than  this  because  there  are  so  many  factors  which  vary  from 
time  to  time  and  in  a  way  which  cannot  be  predicted.  But 
the  fact  is  clear  that  the  period  was  many  times  as  long  as  the 
known  part  of  human  history. 

How  the  ice  sheets  changed  the  land  surface.  —  The  work 
of  glaciers  has  already  been  discussed  in  Chapter  VI.  There  it 
was  shown  that  the  effects  wrought  by  glaciers  are  very  differ- 
ent in  different  places.  Thus  the  last  Canadian  ice  sheets 
produced  varied  changes  according  as  the  country  they  in- 
vaded was  flat,  hilly,  or  mountainous. 

In  the  mountains  of  New  York  the  ice  scoured  off  the  slopes 
of  the  hills,  and  removed  the  crags  and  talus  slopes,  but  did  not 
greatly  change  the  general  forms 
(Fig.  468).  Preexisting  valleys 
were  scoured  out  and  deepened 
where  they  ran  parallel  to  the 
ice  movement,  and  were  par-  *>  468--L°w  mountains  which 

;  have  been  scoured  by  an  ice  sheet, 

tially    filled    With     drift    where        leaving  the  summits  smooth  and 

their    courses    lay   across  the      J?J"Jdec!  lai\d.  *he  valleys  partly 

filled  with  drift. 

line  of  glacial  movement.     The 

so-called  finger  lakes  of  western  New  York  are  in  valleys  thus 

deepened  and  locally  blockaded. 

Where  the  hills  were  lower  and  the  ice  thicker  in  proportion, 
the  effects  of  erosion  by  the  ice  sheet  were  more  pronounced. 
Not  only  was  a  vast  amount  of  soil  and  rock  ground  from  the 
hills,  but  many  of  the  preglacial  valleys  were  completely 
buried  (Fig.  469).  In  such  regions  the  present  hills  and  hol- 
lows are  simply  the  irregularities  of  the  drift  itself,  as  it  was 
deposited.  The  older  topography  has  thus  been  obliterated 
over  large  areas  of  Illinois,  Minnesota,  and  other  northern 
states. 


452 


HISTORICAL  GEOLOGY 


Disturbance  of  river  courses.  —  Before  the  ice  covered  the 
northern  region  the  many  rivers  had  become  in  large  meas- 


FlG.  469.  —  Preglacial  hills  and  valleys  obliterated  by  the  deposition  of 
glacial  till.     (After  Tarr.) 

ure  adjusted  to  the  hard  and  soft  rocks  in  which  they  were 
excavating  their  valleys.     As  the  ice  overspread  their  basins 

many  such  valleys,  with 
their  rivers,  were  wholly 
destroyed,  and  the  new 
streams  which  arose  after 
the  ice  melted  pursue 
courses  quite  unrelated  to 
those  of  their  predeces- 
sors. The  Rock  River  in 
Illinois  and  Wisconsin  ex- 
emplifies this  (Fig.  470). 
Other  streams,  espe- 
cially those  located  near 
the  margin  of  the  ice 
sheet,  were  merely 
crowded  to  one  side  and 
forced  to  make  new  val- 
leys. Thus  the  Missouri 
River  appears  to  have 
been  displaced  by  one  of 
FIG.  470.  —  A  portion  of  northwestern  the  earlier  ice  sheets.  It 

Illinois,  showing  the  course  of  the  Rock 
River  before  and  after  glaciation.  (After 
Leverett.) 


cut  a  new  channel  along 
the  front  of   the   glacier 


THE  QUATERNARY  PERIOD 


453 


(B,  Fig.  471),  and  even  after  the  ice  melted  back  again  the 
river  held  its  new  course. 

Marginal  lakes  of  the  retreat  stage.  —  Like  all  ice  sheets, 
those  of  the  Glacial  epoch  pushed  out  lobes  or  tongues  along 
the  valleys  near  their 
margins.  The  ice  sheet 
thus  came  to  have  scal- 
loped edges.  During  the 
last  retreat  the  great  gla- 
cial lobes  which  had  oc- 
cupied such  depressions 
ponded  the  waters  be- 
tween the  moraines  they 
had  left  and  the  front  of 
the  ice,  thus  producing  a 
series  of  lakes  (Fig.  473). 
The  overflow  water  from 
these  lakes  ran  south- 
ward, largely  into  tribu- 
taries of  the  Mississippi 
River,  —  Lake  Superior 
draining  out  past  Duluth 
and  Lake  Michigan  past 
Chicago.  As  the  ice  re- 
treated slowly  northward  the  lakes  grew  in  size  and  some  joined 
those  next  to  them  to  form  larger  lakes ;  while  others,  having 
lost  the  ice  wall  on  one  side,  disappeared  entirely.  Our  present 
Great  Lakes  began  as  marginal  waters  of  this  kind,  and  it  was 
only  after  the  ice  had  retreated  into  Canada  that  they  were  all 
connected  and  found  the  St.  Lawrence  Valley  the  lowest  point 
of  outflow. 

As  the  ice  retreated  from  Minnesota,  the  Dakotas,  and 
Manitoba,  it  left  a  shallow  basin  in  which  another  great  lake 
came  into  existence.  Lake  Agassiz,  as  it  is  called,  was  once 
five  times  as  large  as  Lake  Superior,  but  when  the  ice  sheet 
which  blocked  its  northern  edge  finally  melted  away,  the 


FIG.  471. — A  and  B.  Diagrammatic  maps 
of  South  Dakota,  showing  how  the  Mis- 
souri River  was  displaced  by  the  invasion 
of  an  ice  sheet.  (Modified  after  Todd.) 


454 


HISTORICAL  GEOLOGY 


waters  of  the  lake  were  drained  off,  leaving  only  much  smaller 
lakes,  as  Winnipeg,  in  the  deepest  parts  of  its  basin.  Its  exist- 
ence is  now  known  from  the  many  terraces  and  sandy  beaches 
made  by  its  waves,  and  by  the  broad,  flat  bottom  built  of  fine 
silts  which  were  deposi- 
ted in  the  lake.  This 
alluvial  plain  is  now  one 
of  the  richest  wheat- 
growing  districts  in  the 
world. 

Features  of  the  latest 
drift  sheet.  —  We  have 
already  said  that  the 
older  sheets  of  drift  have 
been  trenched  by  many 
valleys,  so  that  the  origi- 
nal moraines  and  other 
purely  glacial  features 
are  no  longer  easily  rec- 
ognized. The  last  ice 
sheet  (called  the  Wiscon- 
sin) disappeared  so  re- 
cently that,  in  general, 
erosive  agencies  have 
not  had  time  to  mar  the 
surface  of  the  deposits 
which  it  left. 

Where  the  edge  of  the 
ice  sheet  lingered  we  now  find  terminal  moraines.  There  the 
drift  is  usually  thicker  than  elsewhere,  and  rough  hills  alter- 
nating with  undrained  hollows  are  characteristic.  Many  of 
the  hills  are  composed  of  rudely  stratified  gravel  heaped  up 
in  conical  form.  These  kames  are  often  excavated  for  road 
material  and  railroad  ballast.  On  account  of  the  roughness 
and  bowldery  soil  of  the  terminal  moraines,  they  are  not  com- 
monly cultivated,  but  are  left  as  woodland  and  pastures. 


FIG.  472.  —  A  modern  glacier  on  the  coast 
of  Alaska,  showing  a  marginal  lake  in- 
closed by  a  terminal  moraine  which  is  in 
turn  fringed  by  an  outwash  plain.  (Mod- 
ified after  Maddren,  U.S.  Geol.  Surv.) 
Is  the  glacier  retreating  or  advancing  ? 


THE  QUATERNARY  PERIOD 


455 


Lakes  are  especially  abundant  in  the  terminal  moraines.  In 
Minnesota  and  Wisconsin  thousands  of  them  mark  the  posi- 
tions of  these  .belts. 

Stretching  southward  from  the  moraines,  gently  sloping 
plains  mark  the  outwash  deposits  which  were  built  by 
the  overloaded  glacial  streams.  Owing  to  the  porous,  well- 


FIG.  473.  —  Lobate  edge  of  the  American  ice  sheet  with  marginal  lakes  left 
during  its  retreat.     (Modified  after  Taylor  and  Leverett,  U.S.  Geol.  Surv.) 

drained  soil,  some  of  these  plains  make  excellent  farming 
land,  although  others  are  too  sandy.  Down  every  valley 
leading  away  from  the  moraines,  gravel  and  silt  were  strewn, 
forming  a  flood  plain.  When  the  glaciers  disappeared  and 
the  streams  became  relatively  free  from  detritus  they  were 
able  to  cut  down  into  these  valley  trains  and  have  left  portions 
of  them  as  terraces. 


456 


HISTORICAL  GEOLOGY 


Back  of  the  terminal  moraines,  over  wide  areas,  the  ground 
moraine  prevails,  —  an  undulating  plain  with  gentle  slopes. 
Lakes  and  marshes  strung  on  crooked,  aimless  streams  are  of 
common  occurrence  (Fig.  474).  Where  the  drift  is  thin,  rock 
hills  may  protrude,  their  rounded  forms  and  polished,  grooved 
surfaces  showing  plainly  the  wear  of  the  ice  sheet  upon  them. 
Elsewhere  the  entire  surface  is  molded  from  the  glacial 
bowlder  clay.  In  such  districts  there  may  be  drumlins,  — 
smooth,  elliptical  hills  of  till  all  trending  parallel  to  the  direc- 
tion in  which  the  ice  was  moving. 

The  successive  advances  and  retreats  of  the  ice  made  the 
distribution  of  these  several  features  less  simple  than  might  be 


FIG.  474.  —  Aimless  drainage  of 
a  glaciated  region,  eastern 
Wisconsin. . 


FIG.  475.  —  Tree-shaped  drain- 
age systems  in  an  unglaciated 
region,  northeastern  Iowa. 


expected.  Readvancing  ice  plowed  over  and  defaced  drumlins 
and  moraines  which  had  been  left  at  an  earlier  stage.  Out- 
wash  deposits  of  stratified  drift  made  in  front  of  such  an 
advancing  glacier  were  often  worked  over  and  buried  under  a 
sheet  of  till.  Later,  outwash  sands  and  gravels  were  spread 
over  moraines  as  the  ice  retreated.  The  escaping  water  ponded 
behind  terminal  moraines  cut  channels  in  them  here  and  there. 
Some  of  the  effects  of  glaciation  on  human  affairs.  —  Men- 
tion has  already  Ipeen  made  of  the  excellent  soils  usually  found 
iipdh  glacial  lake  floors  and  outwash  plains.  The  finely 
pulverized  rock  material  left  generally  over  the  glaciated 


THE  QUATERNARY  PERIOD  457 

regions  is  on  the  average  a  better  soil  than  the  residual  sandy 
clays  which  are  produced  by  the  ordinary  weathering  of  many 
rocks.  In  some  places,  notably  in  parts  of  New  England  and 
eastern  Canada,  however,  the  till  contains  so  many  bowlders 
that  cultivation  of  the  soil  is  very  laborious.  Among  the  best 
harbors  in  the  United  States  are  the  glacial  fiords  and  bays  of 
the  New  England  coast.  These  facilities  early  helped  to  lead 
the  people  of  the  region  to  engage  in  fishing  and  to  become 
the  best  seamen  and  shipbuilders  of  the  country. 

The  general  derangement  of  rivers  by  the  ice  sheets  hindered 
inland  navigation  in  a  measure,  but  at  the  same  time  it  con- 
ferred large  benefits  in  the  form  of  available  water  power  from 
the  many  falls  and  rapids.  The  abundance  of  these  falls 
near  the  centers  of  trade  in  northeastern  United  States  has 
assisted  in  making  that  region  a  great  manufacturing  district. 
As  the  progress  of  invention  makes  it  possible  to  transmit 
electric  power  over  longer  and  longer  distances,  these  falls 
will  be  used  more  extensively ;  and  as  the  fuel  resources  of 
the  country  are  gradually  depleted,  more  and  more  depend- 
ence will  be  placed  on  electricity  from  water  power.  The 
glaciated  regions  are  thus  likely  to  retain  their  interest  and 
importance  in  the  manufacturing  industry. 

THE  GLACIAL  EPOCH  OUTSIDE  OF  THE  ICE  SHEETS 

In  the  rest  of  the  United  States  and  in  other  continents  the 
events  of  the  Quaternary  period  were  much  like  those  of  the 
preceding  Tertiary.  By  the  erosion  of  running  water,  pla- 
teaus were  being  cut  into  hills  and  mountains,  winds  were  carv- 
ing out  the  softer  rocks  in  the  deserts,  and  waves  were  eating 
back  the  rocky  coasts.  Along  low-lying  plains  and  river  bot- 
toms, gravel,  sand,  and  mud  were  strewn ;  while  the  waves  and 
winds  built  barriers  and  sand  dunes  along  the  edges  of  the 
shallow  seas. 

There  is  a  decided  contrast  between  the  conditions  and  ap- 
pearance of  the  recently  glaciated  and  the  unglaciated  parts  of 
the  land  (Figs.  474  and  475).  Over  much  of  the  region  where 


458  HISTORICAL  GEOLOGY 

the  last  ice  sheet  left  its  deposits  there  are  lakes,  marshes, 
aimless  rivers,  waterfalls,  and  scattered  bowlders.  Elsewhere, 
lakes  and  marshes  are  confined  largely  to  the  river  bottoms 
and  seashores ;  waterfalls  are  few ;  the  rivers  are  grouped  in 
branching,  treelike  systems ;  bowlders  from  distant  regions  are 
not  to  be  found ;  and  the  hill  soils  are  chiefly  residual. 

Valley  glaciers  in  the  mountains.  —  In  the  mountains 
to-day  there  are  small  valley  glaciers  wherever  there  is  suffi- 
cient cold  and  snowfall.  In  the  Glacial  epoch  these  were 
larger  than  now  and  vastly  more  numerous.  Only  the  lower 
ranges  in  western  United  States  were  free  from  them.  It  is 
easy  to  identify  the  places  where  these  alpine  glaciers  have 
been  at  work,  long  after  they  have  passed  away,  for  they 
not  only  scoured  and  striated  the  valley  floors,  but  made 
the  original  valleys  U-shaped,  sharpened  the  mountain 
peaks  into  crags  and  pinnacles,  and  built  loop-shaped  mo- 
rainic  ridges  farther  down  the  valleys.  Along  the  aban- 
doned valleys  many  lakes  now  testify  to  the  work  of  the 
ice.  The  wild  scenery  of  the  high,  snowy  ranges  to-day 
is  due  largely  to  the  sculpturing  by  Quaternary  glaciers. 

Great  Quaternary  lakes  of  Utah  and  Nevada.  —  The  basin 
which  lies  between  the  Rocky  Mountains  and  the  Sierra 
Nevada  is  now  arid,  and  most  of  the  rivers  flowing  into  it 
dwindle  away  in  the  desert  soils,  or  feed  salt  lakes  from  which 
no  streams  flow  out.  During  the  Glacial  epoch,  all  of  these 
lakes  were  much  larger  than  now.  Great  Salt  Lake  in  Utah 
is  only  a  remnant  of  a  lake,  called  Bonneville  (Fig.  476), 
which  was  two  thirds  as  large  as  Lake  Superior  and  one 
thousand  feet  deep.  The  former  existence  of  this  great  lake 
is  shown  plainly  by  the  series  of  cliffs  and  terraces  which 
parallel  the  slopes  of  the  adjacent  mountains  (Fig.  477). 
These  terraces  were  made  by  the  waves  on  the  lake.  At 
that  time,  Lake  Bonneville  overflowed  northward  into  the 
Snake  River.  In  lakes  with  outlets  the  water  is  continually 
being  changed  and  so  is  not  allowed  to  become  salty.  Bonne- 
ville was  therefore  a  fresh  lake.  In  western  Nevada  a  series 


THE  QUATERNARY  PERIOD 


459 


of  valleys  was  filled  at  that  time  by  a  most  irregular  lake 
which  has  been  named  Lahontan.     In  the  drier  recent  epoch 


FIG.  476.  —  Quaternary  lakes  of  western  United  States. 


FIG.  477.  —  An  abandoned  shore  line  of  Lake  Bonneville.     (After 
U.S.  GeoL  Suro.) 


460  HISTORICAL  GEOLOGY 

its  water,  like  that  of  Lake  Bonneville,  has  largely  evaporated, 
leaving  several  small,  salty  lakes  and  dry,  flat-bottomed  de- 
pressions covered  with  sand  and  crusts  of  lime.  These  deserts 
were  fatal  to  some  of  the  early  emigrants  to  California  who 
came  overland  from  the  East. 

Decline  in  volcanic  activity.  —  While  volcanoes  were  less 
numerous  in  the  West  during  this  period  than  in  the  Tertiary 
they  were  still  fairly  common,  as  is  attested  by  numbers  of 
small  cinder  cones  among  the  western  mountains  and  high 
plateaus.  In  the  bed  of  old  Lake  Bonneville  several  little 
craters  were  formed  after  the  lake  shrank  to  nearly  its  pres- 
ent size.  In  northern  California  there  is  another  little  cone 
surrounded  by  a  recent  lava  flow  and  a  layer  of  ashes  in 
which  the  stumps  of  trees  killed  by  the  last  eruption  are  still 
standing.  The  great  cones  of  Mts.  Shasta,  Rainier,  and  others 
along  the  Pacific  slope,  which  were  built  largely  during  the 
Tertiary  period,  probably  increased  somewhat  in  size  in  the 
course  of  the  Quaternary  period.  Some,  indeed,  are  thought 
to  have  had  eruptions  within  the  last  few  centuries.  Except 
in  Alaska  and  Mexico,  however,  the  present  is  not  a  time  of 
notable  volcanic  activity  in  western  North  America. 

ANIMALS  OF  THE  GLACIAL  EPOCH 

Mammals  attain  their  modern  state.  —  The  Glacial  epoch 
is  so  recent  geologically  that  the  animals  of  that  time  differ 
but  little  from  those  which  exist  to-day.  Add  to  the  mam- 
mals of  to-day  certain  large  forms  which  were  common  then, 
but  have  since  been  exterminated,  and  we  have  essentially 
the  Quaternary  fauna. 

Migrations  caused  by  glacial  fluctuations.  —  Much  more 
striking  peculiarities  are  found  when  we  compare,  from  the 
standpoint  of  their  distribution,  the  animals  of  the  present 
day  with  those  of  the  Glacial  epoch  just  preceding.  Obvi- 
ously the  effect  of  the  slow  expansion  of  the  ice  sheets  was 
to  crowd  all  animals  and  plants  away  from  the  glaciated  region, 


THE  QUATERNARY  PERIOD 


461 


and  for  the  United  States  that  meant  in  general  southward. 
On  the  other  hand,  as  the  ice  retreated  during  the  milder 
times  between  glaciations,  the  same  forms  of  life  would  be 
invited  by  the  amelioration  of  conditions  to  press  northward. 
In  this  way  probably  several  backward  and  forward  migrations 
were  induced. 

Southward  advance  of  Arctic  life.  —  In  the  strictly  glacial 
times  musk-oxen,  such  as  now  live  in  the  Arctic  regions,  came 
as  far  south  as  Kentucky,  and  herds  of  reindeer  ranged  over 
the  treeless  hills  of  France.  Elephants,  such  as  the  mammoth 


FIG.  478.  —  The  American  mastodon.     (Painted  by  Gleeson,  in  the   U.S. 

Nat.  Mus.) 

and  the  mastodon  (Fig.  478),  and  rhinoceroses,  both  covered 
with  long,  woolly  hair,  were  among  these  Arctic  types.  Even 
their  bodies  with  flesh  and  hide  intact  have  been  found  pre- 
served in  the  frozen  gravels  of  certain  Siberian  rivers. 

Southern  forms  come  north.  —  During  the  genial  intervals 
in  which  the  glaciers  disappeared,  many  southern  animals 

B.  &  B.  GEOL. 27 


462 


HISTORICAL  GEOLOGY 


lived  farther  north  than  now.     In  Europe,  hyenas,   lions, 

hippopotami,  and  other 
African  mammals  reached 
England  and  Belgium.  In 
the  United  States,  at 
some  such  time,  sloths 
and  armadillos,  related  to 
South  American  types, 
frequented  the  southern 
states,  coming  as  far  north 
as  Pennsylvania;  while 
horses  were  abundant  in 
Alaska,  along  with  buffa- 
loes (bisons)  and  elephants. 

First  appearance  of  man.  —  In  the  caves  of  France  and 
some  other  parts  of  Europe,  human  bones  and  implements 
have  been  dug  from  beneath  the  hard  layers  of  lime  carbon- 
ate which  incrust  the  floors  of  caves  generally.  With  them 


FIG.  479.  —  Drawing  of  a  reindeer  on  a 
piece  of  bone,  from  a  cave  in  southern 
Europe.  (U.S.  Nat.  Mus.) 


FIG.  480.  —  Seals   carved  on  a  piece   of  bone  found  in  southern  France. 

(U.S.Nat.  Mus.) 
Are  seals  found  in  that  region  to-day  ? 

are  mingled  the  bones  of  the  mammoth,  reindeer,  hyena,  and 
hippopotamus,  none  of  which  have  lived  in  central  Europe  in 
historic  times,  but  which  were  plentiful  there  during  the 
Glacial  or  Interglacial  epochs.  Doubtless  these  earliest  human 
beings  of  which  we  have  knowledge  lived  in  the  caves,  and 
brought  thither  the  bones  of  these  animals,  which  they  had 
killed  with  the  rude  stone-tipped  spears  and  arrows  now  found 
with  their  skeletons.  They  have  even  left  us  fairly  correct 
pictures  of  the  reindeer,  mammoth,  bison,  and  other  animals 
of  this  time,  drawn  on  ivory  and  slate.  It  is  uncertain  whether 
man  had  reached  America  as  early  as  the  last  glacial  advance, 
for  neither  human  bones  nor  implements  have  been  found  with 


THE  QUATERNARY  PERIOD  463 

remains  of  the  extinct  animals  of  the  glacial  times.     There  is, 
however,  no  proof  that  he  was  not  then  on  the  scene. 

THE  RECENT  EPOCH 

Little  change  since  the  glaciers  passed  away.  —  By  the 
departure  of  the  last  ice  sheet  northern  North  America  was 
left  in  very  much  its  present  condition.  Streams  have  cut 
small  valleys  in  the  glacial  drift,  many  lakes  have  been  filled 
by  the  accumulation  of  silt  and  vegetable  matter,  and  some 
have  been  drained;  but  aside  from  such  minor  changes  the 
aspect  given  the  land  by  the  glaciers  has  been  preserved 
through  the  few  thousands  of  years  which  have  elapsed  since 
the  ice  retreated. 

The  Champlain  submergence.  —  About  the  shores  of 
Lakes  Champlain  and  Ontario  marine  shells  and  the  bones  of 
whales  have  been  discovered  in  beds  of  clay  high  above  the 
present  lakes.  In  order  that  the  sea  should  have  extended  in 
so  far,  the  land  must  have  been  several  hundred  feet  lower 
than  now.  At  this  time  the  salt  water  probably  spread  up 
the  Hudson  River  to  Lake  Champlain,  and  also  up  many 
other  valleys  in  the  East.  That  changes  of  level  are  still  in 
progress  is  known  from  the  fact  that  old  beaches  all  along  the 
Great  Lakes  are  no  longer  level,  as  of  course  they  must  have 
been  when  made.  In  general,  they  are  now  higher  on  the 
north  and  northeast  and  are  tilted  southwestward.  Those  of 
Lake  Superior  gradually  sink  from  an  elevation  of  four  hun- 
dred feet  at  the  east  end  of  the  lake  to  lake  level  and  even 
pass  beneath  the  water  before  reaching  Duluth. 

Other  slight  risings  and  sinkings  of  the  land  have  been  in 
progress  recently  in  many  parts  of  the  world.  Indeed,  there 
is  scarcely  a  coast  anywhere  which  does  not  reveal  either 
raised  beaches  and  sea-cut  cliffs,  or  else  drowned  valleys 
and  archipelagos.  The  former  are  conspicuous  at  many 
points  in  California  and  Alaska,  while  the  latter  are  especially 
characteristic  of  the  Atlantic  coast  of  Maine  and  Britain. 


464  HISTORICAL  GEOLOGY 

Final  readjustments  in  the  living  world.  —  Since  the  last 
retreat  of  the  ice,  only  slight  changes  have  been  wrought 
in  the  living  world.  The  animals  and  plants  we  have  to-day 
are  similar  to  those  of  the  Glacial  epoch.  True,  certain  species 
have  migrated  from  one  region  to  another.  Thus  the  reindeer 
has  moved  north  to  Lapland  and  Siberia.  Such  animals  as 
the  mammoth  and  the  cave  bear  have  become  extinct.  Few, 
if  any,  newer  types,  however,  have  appeared. 

The  event  of  chief  importance,  not  only  to  us  as  human 
beings,  but  from  a  purely  geological  viewpoint,  was  the  rapid 
spread  and  advancement  of  the  races  of  men.  Long  before 
the  dawn  of  historic  times  man  had  pushed  outward  from 
the  place  of  his  origin  (itself  yet  unknown)  and  had  colonized 
all  the  larger  lands,  and  eventually  even  such  remote  islands 
as  New  Zealand  and  Hawaii,  whither  no  other  mammal  except 
bats  had  ever  gone.  So  long  ago  were  the  principal  migra- 
tions made  that  the  inhabitants  of  different  continents  have 
become  distinct  races  through  long  isolation.  Some  of  these 
races  have  since  made  comparatively  little  progress,  while 
others  have  increased  and  developed  with  astonishing  rapidity. 

The  geologic  effects  of  human  activities.  —  Probably  no 
other  land  animal,  certainly  no  other  mammal,  has  equaled 
the  human  species  in  its  effect  upon  the  earth  and  its  many 
living  things.  By  digging  canals  he  has  connected  seas  and 
lakes  hitherto  separated.  By  cutting  down  the  forests  he 
has  exposed  to  rain  and  wind  the  soils  formerly  held  firmly 
upon  the  hills.  Clear  streams  have  thus  become  muddy, 
and  permanent  streams  intermittent,  while  springs  have 
disappeared  and  shifting  sands  have  buried  plains  once  fertile. 

In  an  even  more  striking  way  man  has  produced  changes  in 
the  animal  and  plant  world.  Certain  kinds  he  has  protected 
and  domesticated,  so  that  they  have  become  abundant  in 
many  countries.  Others  he  has  hunted  almost  or  quite  to 
extinction.  Among  the  former  are  the  cat,  dog,  and  cattle ; 
while  the  auk,  the  passenger  pigeon,  and  the  bison  may  serve 
as  examples  of  the  latter.  A  full  list  of  either  would  be  long. 


THE   QUATERNARY  PERIOD  465 

Some  he  has  relegated  to  remote  regions ;  thus  the  wild  turkey 
was  formerly  common  throughout  eastern  United  States,  but 
is  now  to  be  seen  only  in  certain  mountainous  portions  of  the 
southern  states.  Still  others  he  has  transported  all  over  the 
world;  for  example,  the  brown  rat,  originally  a  native  of 
northwestern  Europe,  is  now  found  in  every  continent  and 
island  in  the  habitable  zones.  In  more  recent  times  man 
has  even  been  instrumental  in  producing  entirely  new  varieties 
of  animals,  and  especially  of  plants,  by  the  method  of  con- 
trolled breeding,  which  is  now  so  successfully  practiced. 

These  are  but  a  few  examples  of  the  many  changes  of  which 
the  human  races  have  been  the  cause ;  but  they  are  enough 
to  show  how  very  important  the  geologic  and  biologic  influ- 
ence of  this  highest  of  the  mammals  has  become. 

QUESTIONS 

1.  In  the  shape  of  the  edge  of  the  last  ice  sheet,  what  evidence 
is  there  of  the  former  existence  of  a  large  valley  where  Lake  Michigan 
now  lies  ? 

2.  Part  of  this  valley  is  now  below  sea  level.     To  what  extent 
do  rivers  erode  their  valleys  below  sea  level  ?     What  other  factors 
may  have  been  important  here  ? 

3.  In  Indiana   and  Illinois  the  large  bowlders  in  the  drift  are 
chiefly  igneous  and  metamorphic  rocks,   such  as   granite,   gneiss, 
gabbro,  and  quartzite,  while  bowlders  of  the  sandstone  and  lime- 
stone which  underlie  the  drift  are  less  common.     Why  should  this 
be  true? 

4.  Have  the  uplands  of  Figure  477  been  glaciated  ?  The  evidence  ? 

5.  At  a  point  on  the  edge  of  the  Wasatch  Mountains  in  Utah 
a  glacial  moraine  has  been  found  dislocated,  as  shown  in  Figure 
481.     What  events  are  indicated? 

6.  In  northeastern  California  trees  have  been  found  associated 
with  a  bed  of  fine  volcanic 

ash  in  the  relations  shown  in 
Figure  482.  What  inferences 
may  be  drawn  from  this  ? 

7.  Why  should  the  skele- 
tons of  mastodons  and  other 
large  animals  of  the  Glacial 

epoch  be  found  in  peat  bogs  ?    FIG.  481.  —  Dislocated  terminal  moraine. 


£66 


HISTORICAL  GEOLOGY 


8.   Elephants  are   at   present   confined  to  the  tropical  region. 

Do  the  bones  of  elephants 
in  Alaska  and  northern  Si- 
beria therefore  indicate  a 
tropical  climate  there  in  re- 
cent times  ? 

9.  Of  the  caves  with  rela- 
tions as  indicated  in  Figures 
483  and  484,  which  affords 

the   better  evidence  of    the 
FIG    482. -Relations  of  trees  to  a  bed  of    Q1     { ^  f    ^      ^ 

volcanic  ash  near  Lassen  Peak,  California. 

species,  and  why  ? 

10.   Applying    the  principles    already    learned    from    the   past 
history  of  animals,  how  can  you  account  for  the  fact  that  the 


FIG.  483.  —  Section    of    a    cave    in     FIG.  484.  —  Section  of  a  cave  in  lime- 


limestone,  showing  earth  (A)  con- 
taining bones  of  men  and  extinct 
mammals,  overlain  by  a  crust  of 
stalagmite  (B),  and  the  mouth  of 
the  cave  closed  by  a  deposit  of  till. 

Australian  race  of  man  has  made 
less  advancement  than  any  other? 
11.  In  California  the  Quater- 
nary glaciers  were  abundant  on 
mountains  eight  to  ten  thousand 
feet  high,  while  in  Nevada  the 
only  peaks  which  had  even  small 
glaciers  were  more  than  eleven 
thousand  feet  high.  Why  should 
there  be  this  contrast? 


stone,  the  floor  of  which  is  covered 
with  earth  (A)  containing  human 
bones  and  implements. 


FIG.  485.  —  Map  of  the  distribution 
of  bowlders  scraped  off  from  an  out- 
crop of  igneous  rock  by  a  glacier. 


12.  In  a  locality  in  England  bowlders  derived  from  a  small 
volcanic  plug  of  peculiar  rock  are  found  distributed  in  the  glacial 
drift  as  shown  in  Figure  485.  Explain  the  fan-shaped  distribution 


INDEX 


Abrasion,  88. 
Acidic  rock,  24. 
Agents,  geologic,  10. 
Aggradational  processes,  11. 
Algae,  293. 

Algonkian  period,  322. 
Alluvial  fan,  172,  175,  176. 
Alluvial  plain,  176. 
Alluvial  soil,  17. 
Alluvial  terraces,  183. 
Ammonite,  399,  426. 
Amoeba,  295. 

Amphibians,  301,  374,  386. 
Amygdules,  32. 
Angiosperm  leaves,  420. 
Angio sperms,  294. 
Animals,  294. 

Ordovician,  346. 
Animal  groups,  Ordovician,  343. 
Animikean  system,  323. 
Anorthosite,  30. 
Anthracite,  380. 
Anticlines,  66. 
Appalachia,  332,  398. 
Appalachian  Mountains,  398. 
Appalachian  trough,  390. 
Archaean  system,  317. 
Archseopteryx,  412. 
Archseozoic  era,  317. 
Arizona,  326. 
Artesian  wells,  112. 
Arthropods,  299,  356. 
Atmosphere,  13. 

work  of,  86. 
Atoll,  260. 
Augite,  21. 

Bacteria,  293. 
Bad  Lands,  436. 
Bar,  248. 


Barrier  island,  248. 
Barrier  reef,  260. 
Basal   unconformity,  33. 
Basalt,  29. 
Basalt-porphyry,  29. 
Base  level,  140. 
Basic  rocks,  24. 
Basin,  drainage,  142. 
Basins,  glaciated,  219. 
Batholiths,  51. 
Bed,  54. 

Bedding  plane,  18,  54. 
Beds,  limestone,  64. 
Beheaded  stream,  169. 
Belt,  cementation,  80. 

weathering,  80. 
Bergschrund,  200. 
Birds,  301. 

Cretaceous,  426. 

Jurassic,  412. 
Bituminous  coal,  380. 
Bivalves,  298. 
Blastoid,  297,  373. 
Block  mountains,  276. 
Blowhole,  243. 
Bombs,  33. 

Rrachiopods,  298,  336,  343,  354, 
364,  377,  385,  399. 

progress  of,  343. 
Braided  rivers,  178. 
Breakers,  236. 
Breccia,  38. 

volcanic,  33. 

Bridge,  natural,  118,  244. 
Bryophytes,  293. 
Buttes,  167. 
Bysmalith,  51. 

Calcareous  springs,  111. 
Calcite,  22. 
467 


468 


INDEX 


Cambrian  period,  331. 
Cambrian  strata,  331. 
Canons,  165. 
Carbonation,  103. 
Carboniferous  period,  369. 
Carboniferous  swamp,  384. 
Carnivores,  442. 
Cave,  sea,  243,  245. 
Cementation,  belt  of,  80. 

zone  of,  119. 
Cenozoic  era,  306. 
Cephalopods,  299,  346,  355,  365. 
Cetaceans,  442. 
Chalk,  39. 

Chambered  mollusks,  299. 
Champlain  submergence,  463. 
Chemical  elements,  19. 
Chemung  formation,  361. 
Chert,  39. 

Chimney  island,  244. 
Cinders,  33. 

Circumdenudation,  281. 
Circumerosion,  281. 
Cirques,  221. 
Clastic  sediment,  369. 
Clay,  red,  260. 
Cleavage,  20. 
Cliff,  sea,  241, 243. 

undercut,  265. 
Climate,  Cretaceous,  424. 

Pennsylvanian,  386. 

Permian,  393. 

Quaternary,  448. 

Tertiary,  437. 
Clinton  formation,  350. 
Coal  fields,  381. 
Coal,  origin  of,  378. 
Coal  Measures,  377. 

in  Europe,  382. 

in  United  States,  380. 
Coastal  plain,  414. 
Coelenterates,  296. 
Columnar  structure,  53. 
Comanchean  period,  414-416. 
Competent  strata,  67. 
Concretions,  121. 
Cones,  173. 

volcanic,  46. 


Conglomerate,  38. 
Consequent  falls,  159. 
Consolidation,  37. 
Continental  glacier,  207. 
Contour  interval,  93. 
Copper,  120,  325. 
Copper  vein,  80. 
Coral,  259,  296,  345,  353,  366. 
Cordaites,  385. 
Corrasion,  125,  133. 
Cracks,  mud,  56. 

sun,  56. 
Creep,  114. 
Cretaceous  period,  418. 

birds  of,  426. 

climate  of,  424. 

deposits  of,  420,  423. 

inundation  of,  419. 

mammals  of,  431. 

sea  animals  of,  424. 
Crevasse,  199. 
Crinoids,  297,  344,  372. 
Critical  temperature,  81. 
Cross-bedded  sandstone,  54  ;  89. 
Cross-bedding,  54,  55. 
Crumpling,  65. 

in  Jurassic  period,  408. 

in  Permian  period,  390. 
Crust,  earth,  61. 
Crustaceans,  300. 
Crustal  disturbances,  Cretaceous, 
427. 

Ordovician,  346. 
Current,  littoral,  246. 

shore,  246. 
Cut-off,  180. 
Cuttlefish,  299. 
Cycad,  403. 
Cycle,  of  erosion,  150. 

metamorphic,  83. 
Cystid,  297. 

Dead  Sea,  356. 

Deep,  233. 

Degradational  processes,  11. 

Dells,  162. 

Delta,  184,  228. 

Deposition,  91. 


INDEX 


469 


Deposition,  causes  of,  171. 

by  glaciers,    208. 

by  ground  water,  119. 
Deposits,  coral,  259. 

character  of,  256. 

Cretaceous,  420,  423. 

Jurassic,  407. 

in   lakes,    268. 

land-derived,  255. 

organic,  258. 

Tertiary,  433,  435. 
Deserts  in  Triassic  period,  397. 
Devonian  period,  fishes,  365,  367. 

land  life,  368. 

mollusks,  364. 

in   the   East,  359. 

sea  life,  361. 

in  the  West,  359. 
Diastrophism,  11. 
Diatom  ooze,  259. 
Dike,  porphyry,  50. 
Dikes,  diagram  of,  58. 
Dinosaur,  409,  410,  425. 
Diorite,  29,  30. 
Dip,  66. 

Distributaries,  173. 
Divide,  141. 

shifting,  156. 
Dolerite,  29. 
Dolomite,  39. 
Downthrow,  72. 
Drainage  basin,  142. 
Drift,  17,  204,  217. 

unstratified,  205. 
Drumlin,  212. 
Dunes,  95,  97,  98. 
Dust,  volcanic,  33 ; 
Dust  wells,  201. 

Earth,  crust  of,  61. 

history  of,  289. 

origin  of,  308. 
Echinoderms,  297. 
Echinoids,  297,  423. 
Effect  of  ice  sheets  on  land,  451. 
Elements,  chemical,  19. 
Eocene  epoch,  433. 
Eohippus,  444. 


Eolian  sandstone,  98. 
Eolian  soil,  17. 
Epicontinental  sea,  339. 
Erosion,  90,  125,  155,  239. 

cycle  of,  150. 

glacial,   217. 

rate  of,  239. 

stream,  135. 
Esker,  228. 

Exfoliate  weathering,  101. 
Exfoliation,  100. 
Expansion  of  Ordovician  sea,  339. 

Falls,  157, 161. 

consequent,  159, 

subsequent,  159. 
Fan,  187. 

alluvial,  176. 
Faulted  mountains,  276. 
Faults,  70. 

horizontal,  74. 

normal,  71,  73. 

reversed,  71,  72,  73. 
Feeding  grounds,  196. 
Feldspathic  rocks,  29. 
Feldspars,  21. 
Felsite,  29. 
Ferns,  293,  383. 
Fiord,  224. 
Fishes,  300. 

Devonian,  365,  367. 
Fissility,  70,  72. 
Fissures,  47. 
Flint,  39. 

Flood  plain,  176,  182. 
Flood-plain  lakes,  179. 
Flood  tide,  236. 
Flowage,  zone  of,  78. 
Folded  mountains,  153, 277. 
Folded  strata,  67. 
Folds,  66-70. 
Formation,  54. 

Niagara,  351. 
Fossiliferous  slate,  44. 
Fossils,  10,  291,  302. 

of  Proterozoic  era,  329. 

Silurian,  354. 
Fractures,  70-75. 


470 


INDEX 


Fractures,  zone  of,  78. 
Fragmental  rocks,  39. 
Fringing  reef,  259. 
Fungi,  293. 
Fusulina  limestone,  382. 

Gabbro,  29,  82. 

Gastropods,  298,  337,  345,  354 

423. 

Generalized  type,  393,  443. 
Geodes,  122. 
Geologic  divisions,  306. 
Geologic  processes,  10. 
Geology,  9. 
Geysers,  113. 

Glacial  conditions  in  tropics,  394, 
Glacial  trough,  221. 
Glaciated  hills,  219. 
Glaciation,  effects  of,  456. 
Glaciers,  191-230. 
Glass,  volcanic,  31. 
Glassy  rocks,  26. 
Glauconite,  258. 
Globigerina  ooze,  258. 
Gneiss,  18,  42,  43. 
Gold,  120,  437. 
Goniatites,  365,  373. 
Gorges,  162. 
Gradation,  11. 
Grade,  135. 
Gradient,  134. 
Granite,  23,  28. 
Granite  rocks,  130. 
Granitoids,  25. 
Graptolites,  296,  344. 
Great  Plains,  419,  421. 
Great  Salt  Lake,  356. 
Greensand,  258. 
Ground  water,  107. 
Gully,  138. 
Gymnosperms,  294. 
Gypsum,  22. 

Hamilton  shales,  360. 
Hanging  valley,  222,  225. 
Hardpan,  209. 
Helderberg  limestone,  359. 
Hematite,  22,  350, 


Hesperornis,  427. 
Hills,  glaciated,  219. 
Hogbacks,  166,  432. 
Hook,  247. 
Horizons,  331. 
Horizontal  fault,  74. 
Hornblende,  21. 
Huronian  system,  323. 
Hydration,  104. 
Hydroids,  296. 
Hydrosphere,  14,  15,  109. 

Ice  cap,  194,  449. 
Ice  field,  191. 
Ice  pillars,  200. 
Ice  sheet,  194. 

Greenland,  203. 

latest,  455. 

south  polar,  202. 
Ice  sheets,  effect  of,  450^52. 
Ichthyosaur,  401. 
Igneous  rocks,  23,  37,  46,  49. 
[ncompetent  folds,  68. 
tncompetent  strata,  67. 
[nlet,  250. 
[nterior  sea,  351. 
[ntermediate  rocks,  24. 
[ntermittent  streams,  139. 
[ntrenched  meanders,  152. 
intrusion,  laccolithic,  51. 
inundations,  Cretaceous,  419. 
invertebrates,  Triassic,  399. 
Iron,  120,  323. 
sland,  chimney,  244. 

land-tied,  248. 
slands,  barrier,  248. 

ellyfish,  296. 
oints,  53,  70,  71. 
urassic  period,  405-411. 

Kame,  228,  455. 
kaolin,  22. 

£arst  topography,  118. 
^eweenawan  system,  324. 

jaccolithic  intrusion,  51. 
-/accoliths,  50. 
jagoon,  250. 


INDEX 


471 


Lahontan,  459. 
Lake,  233,  261. 
Lake  Bonneville,  459. 
Lakes,  deposits  in,  268. 

extinct,  267. 

fate  of,  266. 

flood-plain,  179. 

function  of,  264. 

marginal,  454. 

ox-bow,  180. 

permanent,  263. 

processes  in,  264. 

Quaternary,  458. 

salt,  268. 

Lake  Superior  region,  322. 
Laminae,  54. 

Lamination,  oblique,  55. 
Lamp  shells,  298. 
Land     animals,     Pennsylvanian, 

385. 
Land,  in  Jurassic  period,  405. 

of  Permian  period,  390. 
Land  life,  Devonian,  368. 
Landslide,  115,  116, 117. 
Land-tied  islands,  248. 
Lapilli,  33. 

Laramie  formation,  423. 
Lava,  25. 
Lava  flow,  49. 
Lava  plateaus,  47. 
Lava  sheet,  58. 
Lead,  120. 

Lead  deposits,  Ordovician,  342. 
Lee  slopes,  219. 
Lepidodendron,  383. 
Levee,  natural,  177. 
Limestone,  39. 

beds  of,  64. 

Mississippian,  370. 
Limonite,  22. 
Lithosphere,  14,  16. 
Littoral  current,  246. 
Load  of  stream,  132. 
Loess,  98. 
Loop,  248. 

Magnetite,  22. 
Mammals,  301,  402 


Mammals,  Cretaceous,  431. 

evolution  of,  444. 

hoofed,  443. 

Jurassic,  411. 

migration  of,  445. 
Man,  appearance  of,  462. 
Mantle  rock,  17. 
Marble,  44. 
Marginal  lakes,  453. 
Marine  strata,  397. 
Marl,  269. 

Massive  structure,  52. 
Mastodon,  461. 
Mauch  Chunk  shale,  370. 
Meanders,  152,  178,  179. 
Medina  sandstone,  350. 
Medusas,  296. 
Mesas,  52,  167. 
Meta-igneous  series,  45. 
Metamorphic  cycle,  83. 
Metamorphic  rock,  19,  41. 
Metamorphism,  11. 
Meta-sedimentary  series,  44. 
Mica,  22. 

Migration  of  dunes,  94. 
Migrations  in  glacial  epoch,  460. 

of  Tertiary  mammals,  445. 
Millstone  grit,  387. 
Mineralizers,  26. 
Mineralogy,  9. 
Minerals,  19,  20. 
Mineral  vein,  79. 
Miocene  epoch,  433. 
Mississippian  crinoids,  372. 
Mississippian  limestone,  378. 
Mississippian  period,  369. 
Mississippian  seas,  371. 
Mississippian  sedimentation,  370. 
Mollusks,  298,  406. 

Devonian,  364. 
Monadnocks,  150. 
Monoclines,  66. 
Monroe  strata,  357. 
Moraines,  209. 
Mosasaur,  425. 
Moss,  293. 

Mountain    growth    of    Tertiary 
period,  438, 


472 


INDEX 


Mountain  range,  275. 
Mountain  system,  275. 
Mountain,  volcanic,  52,  280. 
Mountains,    274. 

block,  276. 

combination,  281. 

destruction  of,  282. 

distribution  of,  275. 

faulted,  276. 

folded,  277; 

of  circumdenudation,  281. 

of  circumerosion,  281. 
Movement  of  glaciers,  197. 
Movements  of  sea,  235. 
Mud,  258. 
Mud  cracks,  56. 

Narrows,  162. 
Natural  bridge,  118,  244. 
Natural  levee,  177. 
Nautilus,  299. 
Nebula,  spiral,  312. 
Nebular  theory,  309. 
Necks,  volcanic,  52,  53. 
Neutral  rocks,  24. 
Newark  formation,  398. 
Niagara  fauna,  352. 
Niagara  formation,  351. 
Normal  faults,  71,  73. 
Nunataks,  203. 

Oblique  lamination,  55. 
Obsidian,  25,  31,  32. 
Ocean  deposits,  256. 
Ocean,  offices  of,  234. 

shores  of,  237. 
Oceans,  233. 
Olivine,  22. 

Onondaga  limestone,  360. 
Oolite,  260. 
Ooze,  37,  258. 
Ordovician  deposits,  342. 
Ordovician  period,  339,  341. 
Ore  vein,  80.     . 
Organic  deposit,  258. 
Oriskany  sandstone,  360. 
Ostracoderms,  365,  366, 


Outcrop,  18,  69. 
Outliers,  357. 
Out  wash  plains,  228. 
Overthrusts,  73,  74. 
Overturned  folds,  67. 
Ox-bow  lake,  180. 
Oxidation,  103. 
Oxides,  19. 
Oyster  shell,  406. 

Paleontology,  9. 

Paleozoic  Alps,  371. 

Palisades,  53. 

Peat,  269,  380. 

Pelecypods,  298,  345,  364,  399, 

406,  423. 

Peneplains,  147,  154. 
Pennsylvanian  period,  376. 
Pennsylvanian  plants,  382-384. 
Pennsylvanian  land  animals,  385. 
Peridotite,  29,  30. 
Permian  period,  389-392. 
Petrification,  122. 
Petroleum,  435. 
Petrology,  9. 
Phenacodus,  443. 
Physical  geology,  10. 
Piedmont  glaciers,  194,  202. 
Piedmont  plateau,  414. 
Pitch,  70. 
Pitchstone,  32. 
Pitchstone-porphyry,  33. 
Piracy  of  streams,  169. 
Plains,  285. 

alluvial,  176. 

base-level,  147. 

classes  of,  286. 

flood,  176,  182. 

origin  of,  286. 

outwash,  228. 
Planes,  bedding,  54. 
Planetesimal  theory,  311. 
Plants,  293. 

Comanchean,  416. 

and  animals,  Ordovician,  346c 

Pennsylvanian,  382. 
Plateaus,  284. 

erosion  of,  285. 


INDEX 


473 


Plateau,  lava,  47. 

origin  of,  285. 
Playas,  56. 
Plesiosaur,  401. 
Pliocene  epoch,  433. 
Plucking,  217. 
Plugs,  53. 

Pocono  sandstone,  370. 
Polyps,  296. 
Porphyritic  texture,  24. 
Porphyry,  28. 
Porphyry  dike,  50. 
Pothole,  161.  ' 
Potomac  series,,  415. 
Processes,  geologic,  10. 
Profile,  interrupted,  153. 
Proterozoic  era,  322. 

fossils  of,  329. 

Proterozoic   group,    unconformi- 
ties in,  327. 
Proterozoic  rocks,  325. 
Proterozoic  strata  in  Arizona,  326. 

in  Lake  Superior  region,  322. 
Pteridospermae,  293, 
Pteridophytes,  293,  383. 
Pteropod,  337. 
Pterosaur,  411. 
Pumice,  31. 

Quaternary  lakes,  458. 
Quaternary  period,  448. 
Quaternary  volcanoes,  460. 
Quartz,  21. 
Quartzite,  38,  79. 

Radiolarian  ooze,  259. 
Rainbow  Falls,  158. 
Range,  mountain,  275. 
Rapids,  157. 
Ravine,  138. 
Reef,  barrier,  260. 

fringing,  259. 
Reindeer,  462. 
Rejuvenated  area,  153. 
Relief,  274. 
Relief  model  of  North  America, 

62. 
Reptiles,  301,  393,  400. 


Residual  soil,  17. 
Reversed  faults,  71,  72,  73. 
Ripple  marks,  55. 
River  system,  142. 
Rivers,  braided,  178. 
Rock  structure,  46,  167. 
Rock  terraces,  166. 
Rock  texture,  24. 
Rock  waste,  17. 
Rocks,  20. 

acidic,  24. 

basic,  24. 

classes  of,  17. 

feldspathic,  29. 

fragmental,  39. 

glassy,  26,  31. 

granite,  130. 

igneous,  19,  23,  37,  42,  49. 

intermediate,  24. 

mantle,  17. 

metamorphic,  19,  41. 

neutral,  24. 

relation  of,  44. 

secondary,  41. 

sedimentary,  19,  37,  40. 

stratified,  19. 
Rodents,  442. 
Run-off,  107. 

Salamander,  374. 
Salina  beds,  356. 
Salt  lakes,  268. 
Sand  dune,  92,  93, 
Sandstone,  38. 

cross-bedded,  89. 

Eolian,  98. 
Sapping,  160. 
Scallop  shell,  385. 
Schist,  42,  82. 
Scoria,  31. 

Scoriaceous  texture,  32. 
Sea,  epicontinental,  339. 

movements  of,  235. 
Sea  animals,  Cretaceous,  424. 
Sea  cave,  243,  245. 
Sea  cliff,  241,  243. 
Sea  erosion,  239. 
Sea  expansion,  Ordovician,  339. 


474 


INDEX 


Sea  level,  254. 

Sea  life,  Permian,  392. 

Sea  urchins,  297,  423. 

Seals,  462. 

Seas,  Mississippian,  371. 

Secondary  rocks,  41. 

Secretions,  122. 

Sedimentary  rock,  19,  37,  40. 

structure,  54. 
Sedimentation, Mississippian, 370. 

Ordovician,  340. 
Sediments,  formation  of,  37. 
Seed  ferns,  293. 
Seed  plants,  294. 
Seepage,  110. 
Shale,  39. 
Shark,  367,  374. 
Shelves,  continental,  256. 
Shore  current,  246. 
Shore  lines,  238,  250. 
Shores,  ocean,  237. 
Silicates,  20. 
Sills,  50. 

Silurian  period,  349. 
Silver,  120. 
Sinks,  118. 
Siphuncle,  299. 
Slate,  43,  82. 

fossiliferous,  44. 
Slickensides,  71. 
Slump,  115. 
Snail  group,  298. 
Snow  line,  191. 
Snow  field,  formation  of,  191. 
Soils,  16. 
Solution  by  ground  water,  116. 

zone  of  greatest,  117. 
Source  of  ground  water,  107. 
Spermatophytes,  294,  416. 
Spit,  247. 
Sponges,  295. 
Spores,  293. 
Springs,  110. 

calcareous,  111. 

deep-seated,  113. 

ferruginous,  111. 

hillside,  111. 

medicinal,  110. 


Stack,  244. 
Stalactites,  120. 
Stalagmites,  120. 
Starfish,  297. 
Stock,  51. 
Stoss  slope,  219. 
Strata,  competent,  67. 

folded,  67. 

incompetent,  67. 
Stratification,  37,  54. 

in  a  sand  dune,  93. 
Stratified  rocks,  19. 
Stratum,  54. 
Stream,  beheaded,  169. 

degrading,  134. 

graded,  135. 

intermittent,  139. 
Stream  piracy,  169. 
Streams,  work  of,  125. 
Striate,  217. 
Strike,  69. 

Structural  valleys,  138. 
Structure,  columnar,  53. 

massive,  52. 
Structures  of  sedimentary  rocks, 

54. 

Submergence,  Champlain,  463. 
Subsequent  falls,  159. 
Subsoil,  16. 
Sun  cracks,  56. 
Suture,  299. 
Syenite,  29,  30. 
Synclines,  66. 
System,  mountain,  275. 

Taconic  revolution,  348. 
Talus,  90,  102. 
Teleosts,  416. 
Temperature,  100. 

critical,  81. 
Tepee  buttes,  432. 
Terrace,  188. 

alluvial,  183. 

river,  152. 

wave-built,  241. 

wave-cut,  241. 
Terrigenous  deposits,  256. 
Tertiary  climate,  437. 


INDEX 


475 


Tertiary  deposits,  433,  435. 
Tertiary  mountains,  439. 
Tertiary  period,  433. 
Tertiary  volcanoes,  440. 
Texture,  porphyritie,  24. 

rock,  24. 

scoriaceous,  32. 

vesicular,  31. 
Thallophytes,  293. 
Throw,  71. 
Tides,  236. 

Tools  of  a  river,  133. 
Topographic   divisions  of  North 

America,  63. 
Topography,  karst,  118. 

mature,  146. 

old,  146. 

serrate  mountain,  102. 
Transportation,  86,  125. 

by  glaciers,  208. 

by  streams,  130. 
Transported  soil,  17. 
Triassic  invertebrates,  399. 
Triassic  period,  397. 

Appalachian  Mountains  in,  398. 

deserts  in,  397. 

marine  strata  in,  397. 

volcanic  eruptions  in,  398. 
Trilobite,  336,  343,  352,  365,  373. 
Tropics,  glacial  condition,  394. 
Trough,  glacial,  221. 
Tuff,  33,  120. 
Type,  generalized,  393,  443. 

Unconformities,     in    Proterozoic 
group,  327. 

value  of,  304. 
Unconformity,  75,  76. 

basal,  322. 

of  Comanchean  period,  415. 
Undercut  cliff,  265. 
Undertow,  235. 
Unglaciated  regions,  457. 
Ungulates,  442. 
United  States  coal  fields,  381. 
Upthrow,  71. 

Valley  deepening,  139. 


Valley  flat,  140. 

Valley  glaciers,  194,  211,  458. 

surface  of,  199. 
Valley  lengthening,  141. 
Valley  system,  142. 
Valley  trains,  227. 
Valleys,  137. 

hanging,  222,  225. 

mature,  143. 

old,  143. 

structural,  138. 

tributary,  142. 

young,  143. 
Vein,  mineral,  79. 
Velocity  of  stream,  134. 
Vertebrates,  300. 
Vesicular  texture,  31. 
Volcanic  ash,  33. 
Volcanic  breccia,  33. 
Volcanic  cones,  46. 
Volcanic  dust,  33. 
Volcanic  eruptions,  Triassic,  398. 
Volcanic  glass,  31. 
Volcanic  mountain,  52,  280. 
Volcanic  necks,  52,  53. 
Volcanoes,  Quaternary,  460. 

Tertiary,  440. 
Vulcanism,  11,  77. 

Warping,  64. 

Waterfall,  160. 

Water  gaps,  162. 

Water  table,  107. 

Waters,  ground,  work  of,  107-123. 

Weathering,  125. 

belt  of,  80. 

exfoliate,  101. 

processes  of,  126. 

rate  of,  127. 
Wells,  artesian,  112. 
Worms,  297. 

Zinc,  120. 

Zinc  deposits,  Ordovician,  342. 

Zone,  of  flowage,  78. 

of  fracture,  78. 

of  greatest  cementation,  119. 

of  greatest  solution,  117. 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


AN  INITIAL  FINE  OF  25  CENTS 

WILL  BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE."  THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
DAY  AND  TO  $i.OO  ON  THE  SEVENTH  DAY 
OVERDUE. 


FEB    2 


1955 


DEC    31956 


FB  2  8  1958 


LD  21-100m-8,'34 


LIBRARY 

G 

THE  UNIVERSITY  OF  CALIFORNIA  LIBRARY 


